feat(offline): local game engine (Phase B1)

The offline vs_ai game engine — a faithful TS port of backend/internal/engine that
drives a whole local game with no backend. Composes with the move generator (#188) and
robot strategy (#189) from Phase A; not yet wired into the UI (Phase B2/B3).

- ui/src/lib/localgame/ruleset.ts: static per-variant tile values / bag counts / blank
  count, mirrored from rules.go (offline scoring is self-contained; online uses the
  server alphabet). Pinned by ruleset.parity.test.ts against a Go fixture.
- bag.ts: the tile bag (fill from counts/blanks, draw-from-end, return+reshuffle) on a
  deterministic in-house PRNG — a game replays from its seed, not bit-identical to a
  server game (per plan).
- board.ts: the mutable board, satisfying the validator/generator read view + set().
- engine.ts: LocalGame — deal / play (reusing validate.ts) / pass / exchange / resign,
  scoreless(6) & out-of-tiles end detection, end-of-game rack penalties, winner; mirrors
  game.go. The end-game math is exported as pure functions, pinned against the Go engine
  (engine.parity.test.ts, 9 constructed positions).
- engine.test.ts: a full-loop smoke — two robots play a whole vs_ai game to completion
  via generateMoves + decide, and it is reproducible from the seed.
- backend: movegen now dumps the per-variant rulesets; a new in-package engine emitter
  (endfixture_test.go, env-gated) produces the end-game golden.

Pure additive library code; no runtime behavior change (unused at runtime, bundle unchanged).

backend/cmd/movegen/main.go

@@ -121,6 +121,8 @@ func main() {
 		return
 	}
 
+	emitRulesets()
+
 	buildSample(*out, "en", rules.English(), sampleWordsEN, []genCase{
 		emptyCase("empty-cared", englishRack("caredts", 0), scrabble.Both, false),
 		emptyCase("empty-dogs", englishRack("dogsent", 0), scrabble.Both, false),
@@ -363,6 +365,37 @@ func russianRack(letters string, blanks int) genRack {
 	return genRack{Letters: idx, Blanks: blanks}
 }
 
+// emitRulesets writes the per-variant static ruleset data (tile values, bag counts, blanks,
+// bingo, rack size) the offline engine mirrors in ui/src/lib/localgame/ruleset.ts, so a TS
+// parity test can pin that hand-copied table to the Go rulesets (scrabble-solver/rules).
+func emitRulesets() {
+	type rsFix struct {
+		Size     int   `json:"size"`
+		RackSize int   `json:"rackSize"`
+		Bingo    int   `json:"bingo"`
+		Blanks   int   `json:"blanks"`
+		Values   []int `json:"values"`
+		Counts   []int `json:"counts"`
+	}
+	out := map[string]rsFix{}
+	for _, v := range []struct {
+		name string
+		rs   *rules.Ruleset
+	}{
+		{"scrabble_en", rules.English()},
+		{"scrabble_ru", rules.RussianScrabble()},
+		{"erudit_ru", rules.Erudit()},
+	} {
+		out[v.name] = rsFix{Size: v.rs.Size(), RackSize: v.rs.RackSize, Bingo: v.rs.Bingo, Blanks: v.rs.Blanks, Values: v.rs.Values, Counts: v.rs.Counts}
+	}
+	dir := filepath.Join("ui", "src", "lib", "localgame", "testdata")
+	if err := os.MkdirAll(dir, 0o755); err != nil {
+		log.Fatalf("movegen: mkdir %s: %v", dir, err)
+	}
+	writeJSON(filepath.Join(dir, "rulesets.json"), out)
+	log.Printf("movegen: wrote %s (3 variants)", filepath.Join(dir, "rulesets.json"))
+}
+
 func writeJSON(path string, v any) {
 	data, err := json.MarshalIndent(v, "", "  ")
 	if err != nil {