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/lobby/invitations.go

@@ -49,6 +49,8 @@ type InvitationSettings struct {
 	HintsAllowed   bool
 	HintsPerPlayer int
 	DropoutTiles   engine.DropoutTiles
+	// MultipleWordsPerTurn true selects standard Scrabble; false the single-word rule.
+	MultipleWordsPerTurn bool
 }
 
 // Invitee is one invited player's seat and response.
@@ -214,14 +216,15 @@ func (svc *InvitationService) CreateInvitation(ctx context.Context, inviterID uu
 		return Invitation{}, fmt.Errorf("lobby: new invitation id: %w", err)
 	}
 	ins := invitationInsert{
-		id:              id,
-		inviterID:       inviterID,
-		variant:         settings.Variant.String(),
-		turnTimeoutSecs: int(settings.TurnTimeout / time.Second),
-		hintsAllowed:    settings.HintsAllowed,
-		hintsPerPlayer:  settings.HintsPerPlayer,
-		dropoutTiles:    settings.DropoutTiles.String(),
-		expiresAt:       svc.now().Add(invitationTTL),
+		id:                   id,
+		inviterID:            inviterID,
+		variant:              settings.Variant.String(),
+		turnTimeoutSecs:      int(settings.TurnTimeout / time.Second),
+		hintsAllowed:         settings.HintsAllowed,
+		hintsPerPlayer:       settings.HintsPerPlayer,
+		dropoutTiles:         settings.DropoutTiles.String(),
+		multipleWordsPerTurn: settings.MultipleWordsPerTurn,
+		expiresAt:            svc.now().Add(invitationTTL),
 	}
 	if err := svc.store.insertInvitation(ctx, ins, inviteeIDs); err != nil {
 		return Invitation{}, err
@@ -265,12 +268,13 @@ func (svc *InvitationService) startGame(ctx context.Context, invitationID uuid.U
 		seats[iv.Seat] = iv.AccountID
 	}
 	g, err := svc.games.Create(ctx, game.CreateParams{
-		Variant:        inv.Settings.Variant,
-		Seats:          seats,
-		TurnTimeout:    inv.Settings.TurnTimeout,
-		HintsAllowed:   inv.Settings.HintsAllowed,
-		HintsPerPlayer: inv.Settings.HintsPerPlayer,
-		DropoutTiles:   inv.Settings.DropoutTiles,
+		Variant:              inv.Settings.Variant,
+		Seats:                seats,
+		TurnTimeout:          inv.Settings.TurnTimeout,
+		HintsAllowed:         inv.Settings.HintsAllowed,
+		HintsPerPlayer:       inv.Settings.HintsPerPlayer,
+		DropoutTiles:         inv.Settings.DropoutTiles,
+		MultipleWordsPerTurn: inv.Settings.MultipleWordsPerTurn,
 	})
 	if err != nil {
 		return err
@@ -322,6 +326,8 @@ type invitationInsert struct {
 	hintsPerPlayer  int
 	dropoutTiles    string
 	expiresAt       time.Time
+	// multipleWordsPerTurn false selects the single-word rule.
+	multipleWordsPerTurn bool
 }
 
 // respondResult reports the state after an invitee response.
@@ -335,8 +341,8 @@ func (s *Store) insertInvitation(ctx context.Context, ins invitationInsert, invi
 		ii := table.GameInvitations.INSERT(
 			table.GameInvitations.InvitationID, table.GameInvitations.InviterID, table.GameInvitations.Variant,
 			table.GameInvitations.TurnTimeoutSecs, table.GameInvitations.HintsAllowed, table.GameInvitations.HintsPerPlayer,
-			table.GameInvitations.DropoutTiles, table.GameInvitations.ExpiresAt,
-		).VALUES(ins.id, ins.inviterID, ins.variant, ins.turnTimeoutSecs, ins.hintsAllowed, ins.hintsPerPlayer, ins.dropoutTiles, ins.expiresAt)
+			table.GameInvitations.DropoutTiles, table.GameInvitations.MultipleWordsPerTurn, table.GameInvitations.ExpiresAt,
+		).VALUES(ins.id, ins.inviterID, ins.variant, ins.turnTimeoutSecs, ins.hintsAllowed, ins.hintsPerPlayer, ins.dropoutTiles, ins.multipleWordsPerTurn, ins.expiresAt)
 		if _, err := ii.ExecContext(ctx, tx); err != nil {
 			return fmt.Errorf("insert invitation: %w", err)
 		}
@@ -377,11 +383,12 @@ func (s *Store) loadInvitation(ctx context.Context, id uuid.UUID) (Invitation, e
 		ID:        row.InvitationID,
 		InviterID: row.InviterID,
 		Settings: InvitationSettings{
-			Variant:        variant,
-			TurnTimeout:    time.Duration(row.TurnTimeoutSecs) * time.Second,
-			HintsAllowed:   row.HintsAllowed,
-			HintsPerPlayer: int(row.HintsPerPlayer),
-			DropoutTiles:   dropout,
+			Variant:              variant,
+			TurnTimeout:          time.Duration(row.TurnTimeoutSecs) * time.Second,
+			HintsAllowed:         row.HintsAllowed,
+			HintsPerPlayer:       int(row.HintsPerPlayer),
+			DropoutTiles:         dropout,
+			MultipleWordsPerTurn: row.MultipleWordsPerTurn,
 		},
 		Status:    row.Status,
 		GameID:    row.GameID,