feat: "multiple words per turn" rule for Russian games

Add a per-game rule chosen on New Game for Russian variants (default off = the
single-word rule; on = standard Scrabble). Off, only the main word along the play
direction is validated and scored; perpendicular cross-words are ignored,
including in robot move generation. The rule rides every create and enqueue
request and joins the matchmaking key, so games and auto-match stay one uniform
path; "Russian-only" is a UI affordance (English always sends standard and shows
no toggle).

- Engine: consume scrabble-solver v1.1.0's PlayOptions{IgnoreCrossWords}, threaded
  through engine.Options.MultipleWordsPerTurn -> playOpts() into validate, score
  and generate.
- Backend: thread the flag through game CreateParams/Game + store (games column),
  lobby InvitationSettings + invitation row, and the matchmaker queue key (variant
  + rule); persisted, so a rebuilt-from-journal game keeps it. Baseline migration
  gains multiple_words_per_turn (DB not versioned); jet regenerated.
- Edge: multiple_words_per_turn added to the EnqueueRequest / CreateInvitationRequest
  FlatBuffers tables (Go + TS regenerated) and threaded through the gateway.
- UI: a "Multiple words per turn" toggle on New Game, shown for Russian variants
  only (auto-match and friend invite), default off; English silently sends standard.
- Tests: backend engine/matchmaker; UI unit (gating) + Playwright e2e (solver
  corner-case + GCG fixtures ship in v1.1.0). Docs + PRERELEASE tracker updated.

backend/internal/engine/game.go

@@ -92,6 +92,12 @@ type Options struct {
 	// DropoutTiles is the disposition of a dropped-out player's tiles in a game
 	// with three or more seats; the zero value removes them from play.
 	DropoutTiles DropoutTiles
+	// MultipleWordsPerTurn selects standard Scrabble when true: every cross-word a
+	// play forms must be a valid word and is scored. When false the game uses the
+	// "single word per turn" rule — only the main word is validated and scored and
+	// perpendicular cross-words are ignored. Callers always set this explicitly; the
+	// zero value (false) is the single-word rule.
+	MultipleWordsPerTurn bool
 }
 
 // Game is the in-memory state of a single match and the pure rules engine over
@@ -104,17 +110,18 @@ type Game struct {
 	variant Variant
 	version string
 
-	board        *board.Board
-	bag          *Bag
-	hands        [][]byte // per player, alphabet-index bytes with blankTile for blanks
-	scores       []int
-	toMove       int
-	scorelessRun int
-	over         bool
-	reason       EndReason
-	resigned     []bool       // per seat; a resigned seat is skipped and cannot win
-	dropoutTiles DropoutTiles // disposition of a resigned seat's tiles
-	log          []MoveRecord
+	board         *board.Board
+	bag           *Bag
+	hands         [][]byte // per player, alphabet-index bytes with blankTile for blanks
+	scores        []int
+	toMove        int
+	scorelessRun  int
+	over          bool
+	reason        EndReason
+	resigned      []bool       // per seat; a resigned seat is skipped and cannot win
+	dropoutTiles  DropoutTiles // disposition of a resigned seat's tiles
+	multipleWords bool         // false = single-word rule (perpendicular cross-words ignored)
+	log           []MoveRecord
 }
 
 // New starts a game described by opts over a dictionary from reg. It resolves
@@ -140,16 +147,17 @@ func New(reg *Registry, opts Options) (*Game, error) {
 
 	rs := solver.Rules()
 	g := &Game{
-		solver:       solver,
-		rules:        rs,
-		variant:      opts.Variant,
-		version:      version,
-		board:        board.New(rs.Rows, rs.Cols),
-		bag:          NewBag(rs, opts.Seed),
-		hands:        make([][]byte, opts.Players),
-		scores:       make([]int, opts.Players),
-		resigned:     make([]bool, opts.Players),
-		dropoutTiles: opts.DropoutTiles,
+		solver:        solver,
+		rules:         rs,
+		variant:       opts.Variant,
+		version:       version,
+		board:         board.New(rs.Rows, rs.Cols),
+		bag:           NewBag(rs, opts.Seed),
+		hands:         make([][]byte, opts.Players),
+		scores:        make([]int, opts.Players),
+		resigned:      make([]bool, opts.Players),
+		dropoutTiles:  opts.DropoutTiles,
+		multipleWords: opts.MultipleWordsPerTurn,
 	}
 	for i := range g.hands {
 		g.hands[i] = g.bag.Draw(rs.RackSize)
@@ -157,6 +165,13 @@ func New(reg *Registry, opts Options) (*Game, error) {
 	return g, nil
 }
 
+// playOpts returns the solver play options for this game's rules. Under the single-word
+// rule (multipleWords false) the solver ignores perpendicular cross-words: only the main
+// word is validated and scored, and move generation is not constrained by cross-words.
+func (g *Game) playOpts() scrabble.PlayOptions {
+	return scrabble.PlayOptions{IgnoreCrossWords: !g.multipleWords}
+}
+
 // Play validates and applies the current player's placement of tiles forming a
 // word in direction dir. It scores the play, refills the rack from the bag,
 // advances the turn and may end the game. It returns ErrTilesNotOnRack when the
@@ -170,7 +185,7 @@ func (g *Game) Play(dir scrabble.Direction, tiles []scrabble.Placement) (MoveRec
 	if err := g.checkHolds(player, placementTiles(tiles)); err != nil {
 		return MoveRecord{}, err
 	}
-	move, err := g.solver.ValidatePlay(g.board, dir, tiles)
+	move, err := g.solver.ValidatePlayOpts(g.board, dir, tiles, g.playOpts())
 	if err != nil {
 		return MoveRecord{}, fmt.Errorf("%w: %v", ErrIllegalPlay, err)
 	}
@@ -279,7 +294,7 @@ func (g *Game) ResignSeat(seat int) (MoveRecord, error) {
 // GenerateMoves returns every legal play for the current player's rack, ranked
 // by descending score. It is empty when the player has no legal play.
 func (g *Game) GenerateMoves() []scrabble.Move {
-	return g.solver.GenerateMoves(g.board, g.rackOf(g.toMove), scrabble.Both)
+	return g.solver.GenerateMovesOpts(g.board, g.rackOf(g.toMove), scrabble.Both, g.playOpts())
 }
 
 // Hint returns the highest-scoring legal play for the current player and true,

backend/internal/engine/singleword_test.go

@@ -0,0 +1,48 @@
+package engine
+
+import "testing"
+
+// TestSingleWordRuleWiring confirms Options.MultipleWordsPerTurn reaches the solver. The
+// single-word game ignores perpendicular cross-words, so move generation from a shared
+// position is a superset of the standard game's; the standard game does not relax them.
+func TestSingleWordRuleWiring(t *testing.T) {
+	const seed = 7
+	mk := func(multipleWords bool) *Game {
+		g, err := New(testReg, Options{
+			Variant:              VariantEnglish,
+			Version:              testVersion,
+			Players:              2,
+			Seed:                 seed,
+			MultipleWordsPerTurn: multipleWords,
+		})
+		if err != nil {
+			t.Fatalf("new game: %v", err)
+		}
+		return g
+	}
+	std, single := mk(true), mk(false)
+	if std.playOpts().IgnoreCrossWords {
+		t.Error("standard game must not ignore cross-words")
+	}
+	if !single.playOpts().IgnoreCrossWords {
+		t.Error("single-word game must ignore cross-words")
+	}
+
+	// Play the same opening (the standard game's top move) in both games, then compare
+	// the next player's candidate moves. Both games share the seed, so the next rack is
+	// identical; relaxed (single-word) generation never drops a legal standard move.
+	hint, ok := std.HintView()
+	if !ok {
+		t.Fatal("opening game has no hint")
+	}
+	if _, err := std.SubmitPlay(hint.Tiles); err != nil {
+		t.Fatalf("standard opening: %v", err)
+	}
+	if _, err := single.SubmitPlay(hint.Tiles); err != nil {
+		t.Fatalf("single-word opening: %v", err)
+	}
+	stdMoves, singleMoves := len(std.GenerateMoves()), len(single.GenerateMoves())
+	if singleMoves < stdMoves {
+		t.Errorf("single-word generation produced %d moves, want >= standard %d", singleMoves, stdMoves)
+	}
+}