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/game/service.go

@@ -107,11 +107,12 @@ func (svc *Service) Create(ctx context.Context, params CreateParams) (Game, erro
 		seed = svc.rng()
 	}
 	g, err := engine.New(svc.registry, engine.Options{
-		Variant:      params.Variant,
-		Version:      svc.version,
-		Players:      len(params.Seats),
-		Seed:         seed,
-		DropoutTiles: params.DropoutTiles,
+		Variant:              params.Variant,
+		Version:              svc.version,
+		Players:              len(params.Seats),
+		Seed:                 seed,
+		DropoutTiles:         params.DropoutTiles,
+		MultipleWordsPerTurn: params.MultipleWordsPerTurn,
 	})
 	if err != nil {
 		if errors.Is(err, engine.ErrUnknownVariant) || errors.Is(err, engine.ErrUnknownVersion) {
@@ -125,15 +126,16 @@ func (svc *Service) Create(ctx context.Context, params CreateParams) (Game, erro
 		return Game{}, fmt.Errorf("game: new id: %w", err)
 	}
 	ins := gameInsert{
-		id:              id,
-		variant:         params.Variant.String(),
-		dictVersion:     svc.version,
-		seed:            seed,
-		players:         len(params.Seats),
-		turnTimeoutSecs: int(timeout / time.Second),
-		hintsAllowed:    params.HintsAllowed,
-		hintsPerPlayer:  params.HintsPerPlayer,
-		dropoutTiles:    params.DropoutTiles.String(),
+		id:                   id,
+		variant:              params.Variant.String(),
+		dictVersion:          svc.version,
+		seed:                 seed,
+		players:              len(params.Seats),
+		turnTimeoutSecs:      int(timeout / time.Second),
+		hintsAllowed:         params.HintsAllowed,
+		hintsPerPlayer:       params.HintsPerPlayer,
+		dropoutTiles:         params.DropoutTiles.String(),
+		multipleWordsPerTurn: params.MultipleWordsPerTurn,
 	}
 	if err := svc.store.CreateGame(ctx, ins, params.Seats); err != nil {
 		return Game{}, err
@@ -934,11 +936,12 @@ func (svc *Service) replay(ctx context.Context, pre Game) (*engine.Game, error)
 		return nil, err
 	}
 	g, err := engine.New(svc.registry, engine.Options{
-		Variant:      pre.Variant,
-		Version:      pre.DictVersion,
-		Players:      pre.Players,
-		Seed:         seed,
-		DropoutTiles: pre.DropoutTiles,
+		Variant:              pre.Variant,
+		Version:              pre.DictVersion,
+		Players:              pre.Players,
+		Seed:                 seed,
+		DropoutTiles:         pre.DropoutTiles,
+		MultipleWordsPerTurn: pre.MultipleWordsPerTurn,
 	})
 	if err != nil {
 		return nil, err