feat: on-device move preview (local eval) with network fallback

Score and validate a tentative move on-device instead of a per-arrangement network
round trip. The dawg reader and the validate/score/direction slice of the
scrabble-solver engine are ported to TypeScript (ui/src/lib/dict), pinned
byte-for-byte to the Go engine by a `conformance` CI job (full-dictionary reader
parity plus a battery of plays across every variant and both cross-word rules,
including the inferred orientation). The server stays authoritative — submit_play
re-validates — so the local result is an advisory accelerator only.

- backend: Registry.DictBytes + an authed GET /api/v1/user/dict/{variant}/{version}
  (immutable) streaming the pinned per-game dawg; caddy routes /dict to the gateway.
- gateway: a session-gated /dict edge route proxying it; fetchDict on the transport.
- client: the dictionary loads on game open (low priority so it never starves the
  game on a slow link; aborted at a 5s cap or when leaving the game), is cached in
  IndexedDB (best-effort, self-healing on a rejected blob) and reused across
  sessions; a warm-up overlay covers a cold load, then the network preview is the
  fallback; a bad-connection breaker stops warming after repeated misses; the move
  preview cancels its in-flight request when the tiles change.
- parity generators backend/cmd/{dictgen,validategen} + gated Vitest suites, run in
  CI against the release dictionaries. A hidden debug readout lists the cached
  dictionaries + breaker state, and its reset clears the cache.
- docs: ARCHITECTURE §5, TESTING, UI_DESIGN, FUNCTIONAL (+ru).

backend/cmd/dictgen/main.go

@@ -0,0 +1,193 @@
+// Command dictgen dumps golden parity vectors from the committed dawg
+// dictionaries so the TypeScript dawg reader can be checked byte-for-byte
+// against the authoritative Go dafsa reader.
+//
+// For each *.dawg file it writes, into the output directory:
+//
+//   - <name>.words.bin — every stored word as alphabet-index bytes, in index
+//     order, framed as [1-byte length][length index bytes]. The word at stream
+//     position k has IndexOfB == k.
+//   - <name>.neg.bin — negative lookups (sequences whose IndexOfB is -1), same
+//     framing, to exercise the not-found path at varying depths.
+//   - <name>.meta.json — NumAdded/NumNodes/NumEdges plus the alphabet size, for
+//     a header-parse sanity cross-check on the TS side.
+//
+// It is a development tool (not built into any service), analogous to
+// cmd/jetgen. Run it from the repository root:
+//
+//	go run ./backend/cmd/dictgen -dawg-dir ../scrabble-solver/dawg -out <dir>
+package main
+
+import (
+	"bufio"
+	"bytes"
+	"encoding/json"
+	"flag"
+	"fmt"
+	"math/rand"
+	"os"
+	"path/filepath"
+	"sort"
+	"strings"
+
+	dawg "github.com/iliadenisov/dafsa"
+)
+
+// meta is the per-dictionary sanity payload cross-checked by the TS reader.
+type meta struct {
+	NumAdded int `json:"numAdded"`
+	NumNodes int `json:"numNodes"`
+	NumEdges int `json:"numEdges"`
+	Alphabet int `json:"alphabet"`
+}
+
+func main() {
+	dawgDir := flag.String("dawg-dir", "../scrabble-solver/dawg", "directory holding the .dawg files")
+	outDir := flag.String("out", "", "output directory for the golden files (required)")
+	negCount := flag.Int("neg", 20000, "number of negative lookups to emit per dictionary")
+	flag.Parse()
+
+	if *outDir == "" {
+		fail("-out is required")
+	}
+	if err := os.MkdirAll(*outDir, 0o755); err != nil {
+		fail("mkdir out: %v", err)
+	}
+
+	files, err := filepath.Glob(filepath.Join(*dawgDir, "*.dawg"))
+	if err != nil {
+		fail("glob: %v", err)
+	}
+	sort.Strings(files)
+	if len(files) == 0 {
+		fail("no .dawg files in %s", *dawgDir)
+	}
+
+	for _, f := range files {
+		if err := process(f, *outDir, *negCount); err != nil {
+			fail("%s: %v", filepath.Base(f), err)
+		}
+	}
+}
+
+// process emits the golden files for a single dawg dictionary.
+func process(path, outDir string, negCount int) error {
+	name := strings.TrimSuffix(filepath.Base(path), ".dawg")
+
+	data, err := os.ReadFile(path)
+	if err != nil {
+		return err
+	}
+	finder, err := dawg.Read(bytes.NewReader(data), 0)
+	if err != nil {
+		return fmt.Errorf("read dawg: %w", err)
+	}
+	defer finder.Close()
+
+	// Stream every stored word in index order; keep a decimated sample and the
+	// maximum alphabet index for negative generation.
+	wf, err := os.Create(filepath.Join(outDir, name+".words.bin"))
+	if err != nil {
+		return err
+	}
+	bw := bufio.NewWriter(wf)
+
+	var (
+		count  int
+		maxIx  byte
+		sample [][]byte
+	)
+	finder.EnumerateB(func(index int, word []byte, final bool) int {
+		if !final {
+			return 0 // Continue
+		}
+		if index != count {
+			panic(fmt.Sprintf("%s: enumerate index gap: got %d want %d", name, index, count))
+		}
+		writeWord(bw, word)
+		for _, b := range word {
+			if b > maxIx {
+				maxIx = b
+			}
+		}
+		if count%4 == 0 && len(sample) < 60000 {
+			sample = append(sample, append([]byte(nil), word...))
+		}
+		count++
+		return 0 // Continue
+	})
+	if err := bw.Flush(); err != nil {
+		return err
+	}
+	if err := wf.Close(); err != nil {
+		return err
+	}
+	if count != finder.NumAdded() {
+		return fmt.Errorf("word count %d != NumAdded %d", count, finder.NumAdded())
+	}
+
+	alphabet := int(maxIx) + 1
+
+	// Negatives: mutate sampled real words and keep the ones the reader rejects.
+	nf, err := os.Create(filepath.Join(outDir, name+".neg.bin"))
+	if err != nil {
+		return err
+	}
+	nbw := bufio.NewWriter(nf)
+	rng := rand.New(rand.NewSource(1))
+	neg := 0
+	for neg < negCount && len(sample) > 0 {
+		base := sample[rng.Intn(len(sample))]
+		cand := append([]byte(nil), base...)
+		switch rng.Intn(3) {
+		case 0: // extend by one index
+			cand = append(cand, byte(rng.Intn(alphabet)))
+		case 1: // flip one index
+			if len(cand) > 0 {
+				cand[rng.Intn(len(cand))] = byte(rng.Intn(alphabet))
+			}
+		case 2: // drop the tail and flip the new last index
+			if len(cand) > 1 {
+				cand = cand[:len(cand)-1]
+				cand[len(cand)-1] = byte(rng.Intn(alphabet))
+			}
+		}
+		if finder.IndexOfB(cand) == -1 {
+			writeWord(nbw, cand)
+			neg++
+		}
+	}
+	if err := nbw.Flush(); err != nil {
+		return err
+	}
+	if err := nf.Close(); err != nil {
+		return err
+	}
+
+	m := meta{NumAdded: finder.NumAdded(), NumNodes: finder.NumNodes(), NumEdges: finder.NumEdges(), Alphabet: alphabet}
+	mb, err := json.MarshalIndent(m, "", "  ")
+	if err != nil {
+		return err
+	}
+	if err := os.WriteFile(filepath.Join(outDir, name+".meta.json"), mb, 0o644); err != nil {
+		return err
+	}
+
+	fmt.Printf("%-12s words=%d negatives=%d alphabet=%d nodes=%d edges=%d\n",
+		name, count, neg, alphabet, finder.NumNodes(), finder.NumEdges())
+	return nil
+}
+
+// writeWord frames one index-byte word as [length][bytes].
+func writeWord(w *bufio.Writer, word []byte) {
+	if len(word) > 255 {
+		panic(fmt.Sprintf("word too long to frame: %d", len(word)))
+	}
+	w.WriteByte(byte(len(word)))
+	w.Write(word)
+}
+
+func fail(format string, args ...any) {
+	fmt.Fprintf(os.Stderr, "dictgen: "+format+"\n", args...)
+	os.Exit(1)
+}

backend/cmd/validategen/main.go

@@ -0,0 +1,486 @@
+// Command validategen produces golden conformance fixtures for the TypeScript
+// move validator (ui/src/lib/dict/validate.ts). For each variant it self-plays
+// greedy games with the authoritative scrabble-solver engine to build realistic
+// board positions, then records a battery of candidate plays — the engine's own
+// top move, letter-mutated variants, random scatters and (on the empty board) an
+// off-centre translation — each paired with the ground-truth result of
+// ValidatePlayOpts (legal, score, the words formed). The TS conformance test
+// replays these and must agree exactly.
+//
+// It is a development tool (not built into any service), analogous to
+// cmd/dictgen. Run it from the repository root:
+//
+//	go run ./backend/cmd/validategen -dawg-dir ../scrabble-solver/dawg -out <dir>
+package main
+
+import (
+	"bytes"
+	"encoding/json"
+	"flag"
+	"fmt"
+	"math/rand"
+	"os"
+	"path/filepath"
+
+	"gitea.iliadenisov.ru/developer/scrabble-solver/board"
+	"gitea.iliadenisov.ru/developer/scrabble-solver/rack"
+	"gitea.iliadenisov.ru/developer/scrabble-solver/rules"
+	"gitea.iliadenisov.ru/developer/scrabble-solver/scrabble"
+	"gitea.iliadenisov.ru/developer/scrabble-solver/selfplay"
+	dawg "github.com/iliadenisov/dafsa"
+)
+
+// blankTile marks a blank tile in a drawn hand (matches selfplay).
+const blankTile byte = 0xff
+
+// variantSpec pairs a variant label with its ruleset and dawg file.
+type variantSpec struct {
+	name  string
+	rules *rules.Ruleset
+	dawg  string
+}
+
+// cell is an occupied board square or a placement (alphabet-index letter).
+type cell struct {
+	R, C, Letter int
+	Blank        bool
+}
+
+// word mirrors scrabble.Word in index space.
+type word struct {
+	Row, Col, Dir int
+	Letters       []int
+	Blanks        []bool
+	Score         int
+}
+
+// fixture is one candidate play with the engine's ground-truth verdict.
+type fixture struct {
+	Board            int    `json:"board"` // index into the boards list
+	Dir              int    `json:"dir"`
+	IgnoreCrossWords bool   `json:"ignoreCrossWords"`
+	Tiles            []cell `json:"tiles"`
+	Legal            bool   `json:"legal"`
+	Score            int    `json:"score"`
+	Bonus            int    `json:"bonus"`
+	Main             *word  `json:"main,omitempty"`
+	Cross            []word `json:"cross,omitempty"`
+}
+
+// alphaEntry mirrors one row of the per-variant alphabet table the server sends the
+// client (index, concrete letter as the ruleset emits it, tile value), so the adapter
+// cross-test can drive the letter-space client path exactly as production does.
+type alphaEntry struct {
+	Index  int    `json:"index"`
+	Letter string `json:"letter"`
+	Value  int    `json:"value"`
+}
+
+// variantFile is the whole conformance payload for one variant.
+type variantFile struct {
+	Variant  string       `json:"variant"`
+	Rows     int          `json:"rows"`
+	Cols     int          `json:"cols"`
+	Center   int          `json:"center"`
+	RackSize int          `json:"rackSize"`
+	Bingo    int          `json:"bingo"`
+	Values   []int        `json:"values"`
+	Premiums []int        `json:"premiums"` // row-major rules.Premium codes
+	Alphabet []alphaEntry `json:"alphabet"`
+	Boards   [][]cell     `json:"boards"`
+	Fixtures []fixture    `json:"fixtures"`
+}
+
+func main() {
+	dawgDir := flag.String("dawg-dir", "../scrabble-solver/dawg", "directory holding the .dawg files")
+	outDir := flag.String("out", "", "output directory for the fixture files (required)")
+	games := flag.Int("games", 6, "self-play games per (variant, rule)")
+	plies := flag.Int("plies", 40, "maximum plies captured per game")
+	flag.Parse()
+	if *outDir == "" {
+		fail("-out is required")
+	}
+	if err := os.MkdirAll(*outDir, 0o755); err != nil {
+		fail("mkdir out: %v", err)
+	}
+
+	specs := []variantSpec{
+		{"scrabble_en", rules.English(), "en_sowpods.dawg"},
+		{"scrabble_ru", rules.RussianScrabble(), "ru_scrabble.dawg"},
+		{"erudit_ru", rules.Erudit(), "ru_erudit.dawg"},
+	}
+	for _, sp := range specs {
+		if err := generate(sp, *dawgDir, *outDir, *games, *plies); err != nil {
+			fail("%s: %v", sp.name, err)
+		}
+	}
+}
+
+func generate(sp variantSpec, dawgDir, outDir string, games, plies int) error {
+	data, err := os.ReadFile(filepath.Join(dawgDir, sp.dawg))
+	if err != nil {
+		return err
+	}
+	finder, err := dawg.Read(bytes.NewReader(data), 0)
+	if err != nil {
+		return fmt.Errorf("read dawg: %w", err)
+	}
+	defer finder.Close()
+
+	rs := sp.rules
+	solver := scrabble.NewSolver(rs, finder)
+
+	out := variantFile{
+		Variant: sp.name, Rows: rs.Rows, Cols: rs.Cols, Center: rs.Center,
+		RackSize: rs.RackSize, Bingo: rs.Bingo, Values: rs.Values,
+		Premiums: premiumCodes(rs), Alphabet: alphabetOf(rs),
+	}
+
+	// Capture under both the standard rule and the single-word rule, building the
+	// board with the same rule so positions are reachable under it.
+	for _, ignore := range []bool{false, true} {
+		opts := scrabble.PlayOptions{IgnoreCrossWords: ignore}
+		for g := range games {
+			seed := int64(g*1000) + boolseed(ignore) + variantSeed(sp.name)
+			playAndCapture(&out, rs, solver, opts, seed, plies)
+		}
+	}
+
+	b, err := json.Marshal(&out)
+	if err != nil {
+		return err
+	}
+	if err := os.WriteFile(filepath.Join(outDir, sp.name+".fixtures.json"), b, 0o644); err != nil {
+		return err
+	}
+	fmt.Printf("%-12s boards=%d fixtures=%d\n", sp.name, len(out.Boards), len(out.Fixtures))
+	return nil
+}
+
+// playAndCapture greedily self-plays one game, recording candidate plays against
+// each board position along the way.
+func playAndCapture(out *variantFile, rs *rules.Ruleset, solver *scrabble.Solver, opts scrabble.PlayOptions, seed int64, plies int) {
+	rng := rand.New(rand.NewSource(seed))
+	bag := selfplay.NewBag(rs, seed)
+	b := board.New(rs.Rows, rs.Cols)
+	hands := [2][]byte{bag.Draw(rs.RackSize), bag.Draw(rs.RackSize)}
+
+	passes := 0
+	for turn := range plies {
+		p := turn % 2
+		rk := rackOf(hands[p], rs.Size())
+		moves := solver.GenerateMovesOpts(b, rk, scrabble.Both, opts)
+		if len(moves) == 0 {
+			if passes++; passes >= 4 {
+				break
+			}
+			continue
+		}
+		passes = 0
+		top := moves[0]
+
+		boardIdx := len(out.Boards)
+		out.Boards = append(out.Boards, boardCells(b))
+		captureCandidates(out, rs, solver, opts, b, boardIdx, top, rng)
+
+		scrabble.Apply(b, top)
+		hands[p] = removeUsed(hands[p], top)
+		if need := rs.RackSize - len(hands[p]); need > 0 {
+			hands[p] = append(hands[p], bag.Draw(need)...)
+		}
+		if len(hands[p]) == 0 && bag.Len() == 0 {
+			break
+		}
+	}
+}
+
+// captureCandidates records the engine's top move plus derived candidates for one
+// board, each with its ValidatePlayOpts verdict.
+func captureCandidates(out *variantFile, rs *rules.Ruleset, solver *scrabble.Solver, opts scrabble.PlayOptions, b *board.Board, boardIdx int, top scrabble.Move, rng *rand.Rand) {
+	size := rs.Size()
+	record := func(tiles []scrabble.Placement) {
+		if len(tiles) == 0 {
+			return
+		}
+		out.Fixtures = append(out.Fixtures, makeFixture(solver, opts, b, boardIdx, tiles))
+	}
+
+	// The engine's own top move (legal).
+	record(top.Tiles)
+
+	// Letter-mutated variants: usually reject on the dictionary, occasionally form
+	// a different legal word.
+	for range 3 {
+		mut := clonePlacements(top.Tiles)
+		i := rng.Intn(len(mut))
+		mut[i].Letter = byte((int(mut[i].Letter) + 1 + rng.Intn(size-1)) % size)
+		record(mut)
+	}
+
+	// Random scatters: exercise geometry, dictionary and connectivity paths.
+	for range 3 {
+		record(randomScatter(b, size, 2+rng.Intn(4), rng))
+	}
+
+	// Single tiles abutting the board exercise the direction inference — a single
+	// tile is ambiguous, its orientation resolved from which axis it extends.
+	for range 3 {
+		if t, ok := randomAdjacentSingle(b, size, rng); ok {
+			record([]scrabble.Placement{t})
+		}
+	}
+
+	// On the empty board, an off-centre translation of the first move exercises the
+	// first-move centre rule.
+	if b.IsEmpty() {
+		shifted := clonePlacements(top.Tiles)
+		ok := true
+		for i := range shifted {
+			shifted[i].Row++
+			shifted[i].Col++
+			if !b.InBounds(shifted[i].Row, shifted[i].Col) {
+				ok = false
+				break
+			}
+		}
+		if ok {
+			record(shifted)
+		}
+	}
+}
+
+// makeFixture validates a candidate against board b and serializes it with its
+// ground truth. Word breakdown is recorded only for legal plays (the TS test
+// checks words only then); an illegal play records legal=false alone.
+func makeFixture(solver *scrabble.Solver, opts scrabble.PlayOptions, b *board.Board, boardIdx int, tiles []scrabble.Placement) fixture {
+	// Infer the orientation exactly as the backend evaluate does (dir-less), so the
+	// fixture matches the real eval path and pins the client's ported inference.
+	dir := playDirectionMirror(solver, b, tiles, opts)
+	fx := fixture{
+		Board:            boardIdx,
+		Dir:              int(dir),
+		IgnoreCrossWords: opts.IgnoreCrossWords,
+		Tiles:            placementCells(tiles),
+	}
+	m, err := solver.ValidatePlayOpts(b, dir, tiles, opts)
+	if err == nil {
+		fx.Legal = true
+		fx.Score = m.Score
+		fx.Bonus = m.Bonus
+		fx.Main = toWord(m.Main)
+		for _, cw := range m.Cross {
+			fx.Cross = append(fx.Cross, *toWord(cw))
+		}
+	}
+	return fx
+}
+
+func placementCells(ts []scrabble.Placement) []cell {
+	cs := make([]cell, len(ts))
+	for i, t := range ts {
+		cs[i] = cell{R: t.Row, C: t.Col, Letter: int(t.Letter), Blank: t.Blank}
+	}
+	return cs
+}
+
+func toWord(w scrabble.Word) *word {
+	letters := make([]int, len(w.Letters))
+	for i, l := range w.Letters {
+		letters[i] = int(l)
+	}
+	return &word{
+		Row: w.Row, Col: w.Col, Dir: int(w.Dir),
+		Letters: letters, Blanks: append([]bool(nil), w.Blanks...), Score: w.Score,
+	}
+}
+
+func alphabetOf(rs *rules.Ruleset) []alphaEntry {
+	n := rs.Alphabet.Size()
+	out := make([]alphaEntry, n)
+	for i := range n {
+		ch, _ := rs.Alphabet.Character(byte(i))
+		out[i] = alphaEntry{Index: i, Letter: ch, Value: rs.Values[i]}
+	}
+	return out
+}
+
+func premiumCodes(rs *rules.Ruleset) []int {
+	codes := make([]int, rs.Rows*rs.Cols)
+	for i := range codes {
+		codes[i] = int(rs.PremiumAt(i))
+	}
+	return codes
+}
+
+func boardCells(b *board.Board) []cell {
+	var cs []cell
+	for r := 0; r < b.Rows(); r++ {
+		for c := 0; c < b.Cols(); c++ {
+			if b.Filled(r, c) {
+				v := b.At(r, c)
+				cs = append(cs, cell{R: r, C: c, Letter: int(v&0x3f) - 1, Blank: v&0x80 != 0})
+			}
+		}
+	}
+	return cs
+}
+
+func clonePlacements(ts []scrabble.Placement) []scrabble.Placement {
+	return append([]scrabble.Placement(nil), ts...)
+}
+
+// randomScatter picks n distinct empty in-bounds squares with random letters.
+func randomScatter(b *board.Board, size, n int, rng *rand.Rand) []scrabble.Placement {
+	seen := map[[2]int]bool{}
+	var ts []scrabble.Placement
+	for tries := 0; tries < n*20 && len(ts) < n; tries++ {
+		r := rng.Intn(b.Rows())
+		c := rng.Intn(b.Cols())
+		if seen[[2]int{r, c}] || b.Filled(r, c) {
+			continue
+		}
+		seen[[2]int{r, c}] = true
+		ts = append(ts, scrabble.Placement{Row: r, Col: c, Letter: byte(rng.Intn(size)), Blank: rng.Intn(10) == 0})
+	}
+	return ts
+}
+
+// randomAdjacentSingle picks a random empty in-bounds square abutting at least one
+// filled square, with a random letter — a single-tile play whose orientation the
+// inference must resolve. It returns ok=false on an empty board.
+func randomAdjacentSingle(b *board.Board, size int, rng *rand.Rand) (scrabble.Placement, bool) {
+	var cands [][2]int
+	for r := 0; r < b.Rows(); r++ {
+		for c := 0; c < b.Cols(); c++ {
+			if b.Filled(r, c) {
+				continue
+			}
+			if b.Filled(r-1, c) || b.Filled(r+1, c) || b.Filled(r, c-1) || b.Filled(r, c+1) {
+				cands = append(cands, [2]int{r, c})
+			}
+		}
+	}
+	if len(cands) == 0 {
+		return scrabble.Placement{}, false
+	}
+	rc := cands[rng.Intn(len(cands))]
+	return scrabble.Placement{Row: rc[0], Col: rc[1], Letter: byte(rng.Intn(size)), Blank: rng.Intn(10) == 0}, true
+}
+
+// playDirectionMirror mirrors engine (*Game).playDirection: the geometric
+// resolution, except a single tile under the single-word rule tries both
+// orientations through the solver and keeps the higher-scoring legal one (H wins
+// ties). It reproduces the orientation the backend evaluate infers.
+func playDirectionMirror(solver *scrabble.Solver, b *board.Board, placements []scrabble.Placement, opts scrabble.PlayOptions) scrabble.Direction {
+	geo := resolveDirectionMirror(b, placements)
+	if len(placements) != 1 || !opts.IgnoreCrossWords {
+		return geo
+	}
+	best, found, bestScore := geo, false, 0
+	for _, dir := range [...]scrabble.Direction{scrabble.Horizontal, scrabble.Vertical} {
+		m, err := solver.ValidatePlayOpts(b, dir, placements, opts)
+		if err != nil {
+			continue
+		}
+		if !found || m.Score > bestScore {
+			best, found, bestScore = dir, true, m.Score
+		}
+	}
+	return best
+}
+
+// resolveDirectionMirror mirrors engine.resolveDirection.
+func resolveDirectionMirror(b *board.Board, placements []scrabble.Placement) scrabble.Direction {
+	if len(placements) >= 2 {
+		row := placements[0].Row
+		for _, p := range placements[1:] {
+			if p.Row != row {
+				return scrabble.Vertical
+			}
+		}
+		return scrabble.Horizontal
+	}
+	if len(placements) == 1 {
+		p := placements[0]
+		h := runLengthMirror(b, p.Row, p.Col, scrabble.Horizontal)
+		v := runLengthMirror(b, p.Row, p.Col, scrabble.Vertical)
+		if v >= 2 && v > h {
+			return scrabble.Vertical
+		}
+		if h >= 2 {
+			return scrabble.Horizontal
+		}
+		if v >= 2 {
+			return scrabble.Vertical
+		}
+	}
+	return scrabble.Horizontal
+}
+
+// runLengthMirror mirrors engine.runLength.
+func runLengthMirror(b *board.Board, row, col int, dir scrabble.Direction) int {
+	dr, dc := 0, 1
+	if dir == scrabble.Vertical {
+		dr, dc = 1, 0
+	}
+	n := 1
+	for r, c := row-dr, col-dc; b.Filled(r, c); r, c = r-dr, c-dc {
+		n++
+	}
+	for r, c := row+dr, col+dc; b.Filled(r, c); r, c = r+dr, c+dc {
+		n++
+	}
+	return n
+}
+
+// rackOf builds a generation rack from a hand of tiles (reimplemented from the
+// unexported selfplay helper).
+func rackOf(tiles []byte, size int) rack.Rack {
+	r := rack.New(size)
+	for _, t := range tiles {
+		if t == blankTile {
+			r.AddBlank()
+		} else {
+			r.Add(t)
+		}
+	}
+	return r
+}
+
+// removeUsed returns the hand with the tiles consumed by m removed.
+func removeUsed(tiles []byte, m scrabble.Move) []byte {
+	out := append([]byte(nil), tiles...)
+	for _, p := range m.Tiles {
+		want := p.Letter
+		if p.Blank {
+			want = blankTile
+		}
+		for i, t := range out {
+			if t == want {
+				out = append(out[:i], out[i+1:]...)
+				break
+			}
+		}
+	}
+	return out
+}
+
+func boolseed(b bool) int64 {
+	if b {
+		return 500000
+	}
+	return 0
+}
+
+func variantSeed(name string) int64 {
+	var s int64
+	for _, r := range name {
+		s = s*131 + int64(r)
+	}
+	return s
+}
+
+func fail(format string, args ...any) {
+	fmt.Fprintf(os.Stderr, "validategen: "+format+"\n", args...)
+	os.Exit(1)
+}

backend/internal/engine/registry.go

@@ -21,11 +21,13 @@ var dictFiles = map[Variant]string{
 	VariantErudit:          "ru_erudit.dawg",
 }
 
-// entry is one resident dictionary: the loaded finder and the solver built over
-// it. The finder is retained so Close can release it.
+// entry is one resident dictionary: the loaded finder, the solver built over it
+// and the file it was loaded from. The finder is retained so Close can release
+// it; path is retained so the raw bytes can be re-read for the client download.
 type entry struct {
 	finder dawg.Finder
 	solver *scrabble.Solver
+	path   string
 }
 
 // Registry holds the dictionaries resident in memory, addressed by variant and
@@ -130,7 +132,7 @@ func (r *Registry) Load(v Variant, version, dir string) error {
 	if old, ok := r.entries[v][version]; ok {
 		_ = old.finder.Close()
 	}
-	r.entries[v][version] = entry{finder: finder, solver: scrabble.NewSolver(rs, finder)}
+	r.entries[v][version] = entry{finder: finder, solver: scrabble.NewSolver(rs, finder), path: path}
 	r.latest[v] = version
 	return nil
 }
@@ -202,6 +204,30 @@ func (r *Registry) Versions(v Variant) []string {
 	return versions
 }
 
+// DictBytes returns the raw serialized DAWG for the (variant, version) pair,
+// re-read from the file it was loaded from — the same immutable bytes the solver
+// holds. It backs the client-side dictionary download for the local move
+// preview. It returns ErrUnknownVariant or ErrUnknownVersion when that dictionary
+// is not resident, and wraps any read error. The file is read outside the lock.
+func (r *Registry) DictBytes(v Variant, version string) ([]byte, error) {
+	r.mu.RLock()
+	versions, ok := r.entries[v]
+	if !ok {
+		r.mu.RUnlock()
+		return nil, fmt.Errorf("%w: %s", ErrUnknownVariant, v)
+	}
+	e, ok := versions[version]
+	r.mu.RUnlock()
+	if !ok {
+		return nil, fmt.Errorf("%w: %s/%s", ErrUnknownVersion, v, version)
+	}
+	data, err := os.ReadFile(e.path)
+	if err != nil {
+		return nil, fmt.Errorf("engine: read %s/%s dictionary bytes from %s: %w", v, version, e.path, err)
+	}
+	return data, nil
+}
+
 // Lookup reports whether word is present in the (variant, version) dictionary,
 // backing the unlimited word-check tool. It returns ErrUnknownVariant or
 // ErrUnknownVersion when that dictionary is not resident, and an error when word

backend/internal/game/service.go

@@ -1649,6 +1649,14 @@ func (svc *Service) lookupWord(variant engine.Variant, version, word string) (bo
 	return present, nil
 }
 
+// DictBytes returns the raw serialized dictionary for the (variant, version) pair
+// from the registry, backing the client-side dictionary download used by the
+// local move preview. It surfaces engine.ErrUnknownVariant /
+// engine.ErrUnknownVersion when that dictionary is not resident.
+func (svc *Service) DictBytes(variant engine.Variant, version string) ([]byte, error) {
+	return svc.registry.DictBytes(variant, version)
+}
+
 // hintsRemaining is a player's remaining hint budget: the unspent per-game
 // allowance plus the profile wallet.
 func hintsRemaining(allowance, used, wallet int) int {

backend/internal/server/handlers.go

@@ -90,6 +90,9 @@ func (s *Server) registerRoutes() {
 		u.GET("/games/:id/draft", s.handleGetDraft)
 		u.PUT("/games/:id/draft", s.handleSaveDraft)
 		u.POST("/games/:id/hide", s.handleHideGame)
+		// Raw dictionary download for the client-side local move preview, keyed by
+		// the game's pinned (variant, version); immutable, so cached hard.
+		u.GET("/dict/:variant/:version", s.handleDictBytes)
 	}
 	if s.feedback != nil {
 		u.POST("/feedback", s.handleFeedbackSubmit)

backend/internal/server/handlers_dict.go

@@ -0,0 +1,31 @@
+package server
+
+import (
+	"net/http"
+
+	"github.com/gin-gonic/gin"
+
+	"scrabble/backend/internal/engine"
+)
+
+// handleDictBytes streams the raw serialized dictionary for a (variant, version)
+// pair so the client can validate and score moves locally — the local move
+// preview — instead of a network round trip per tile arrangement. It is reached
+// only through the gateway's session-gated /dict route (which resolves the
+// X-User-ID this group requires), and served with a long immutable cache lifetime
+// because a published dictionary version never changes. An unknown variant or a
+// version that is not resident is a 404.
+func (s *Server) handleDictBytes(c *gin.Context) {
+	variant, err := engine.ParseVariant(c.Param("variant"))
+	if err != nil {
+		c.JSON(http.StatusNotFound, gin.H{"error": "unknown variant"})
+		return
+	}
+	data, err := s.games.DictBytes(variant, c.Param("version"))
+	if err != nil {
+		c.JSON(http.StatusNotFound, gin.H{"error": "dictionary not found"})
+		return
+	}
+	c.Header("Cache-Control", "public, max-age=31536000, immutable")
+	c.Data(http.StatusOK, "application/octet-stream", data)
+}