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,

backend/internal/lobby/matchmaker.go

@@ -14,6 +14,14 @@ import (
 	"scrabble/backend/internal/notify"
 )
 
+// matchKey buckets the auto-match pool: two players are paired only when they chose
+// the same variant and the same per-turn word rule (multipleWords), so a game always
+// starts under a rule both players asked for.
+type matchKey struct {
+	variant       engine.Variant
+	multipleWords bool
+}
+
 // Matchmaker is the in-memory auto-match pool: a FIFO queue per variant that pairs
 // the next two humans into a two-player game, or — when no human arrives within
 // the wait window — substitutes a robot. It holds no database state and is lost on
@@ -35,8 +43,8 @@ type Matchmaker struct {
 	log       *zap.Logger
 
 	mu           sync.Mutex
-	queues       map[engine.Variant][]uuid.UUID
-	queued       map[uuid.UUID]engine.Variant
+	queues       map[matchKey][]uuid.UUID
+	queued       map[uuid.UUID]matchKey
 	waitingSince map[uuid.UUID]time.Time
 	results      map[uuid.UUID]game.Game
 	rng          *rand.Rand
@@ -55,8 +63,8 @@ func NewMatchmaker(games GameCreator, robots RobotProvider, waitDelay time.Durat
 		clock:        func() time.Time { return time.Now().UTC() },
 		pub:          notify.Nop{},
 		log:          log,
-		queues:       make(map[engine.Variant][]uuid.UUID),
-		queued:       make(map[uuid.UUID]engine.Variant),
+		queues:       make(map[matchKey][]uuid.UUID),
+		queued:       make(map[uuid.UUID]matchKey),
 		waitingSince: make(map[uuid.UUID]time.Time),
 		results:      make(map[uuid.UUID]game.Game),
 		rng:          rand.New(rand.NewSource(time.Now().UnixNano())),
@@ -101,34 +109,36 @@ type EnqueueResult struct {
 	Game    game.Game
 }
 
-// Enqueue joins accountID to the variant pool. If an opponent already waits, the
-// two are paired (seat order randomised for first-move fairness) and a game starts
-// immediately; otherwise the account waits, and a later pairing or robot
-// substitution is delivered through Poll. An account already waiting in any pool
-// gets ErrAlreadyQueued.
-func (m *Matchmaker) Enqueue(ctx context.Context, accountID uuid.UUID, variant engine.Variant) (EnqueueResult, error) {
+// Enqueue joins accountID to the auto-match pool for variant under the chosen
+// per-turn word rule (multipleWords). If an opponent already waits for the same
+// variant and rule, the two are paired (seat order randomised for first-move
+// fairness) and a game starts immediately; otherwise the account waits, and a later
+// pairing or robot substitution is delivered through Poll. An account already waiting
+// in any pool gets ErrAlreadyQueued.
+func (m *Matchmaker) Enqueue(ctx context.Context, accountID uuid.UUID, variant engine.Variant, multipleWords bool) (EnqueueResult, error) {
+	key := matchKey{variant: variant, multipleWords: multipleWords}
 	m.mu.Lock()
 	if _, ok := m.queued[accountID]; ok {
 		m.mu.Unlock()
 		return EnqueueResult{}, ErrAlreadyQueued
 	}
-	q := m.queues[variant]
+	q := m.queues[key]
 	if len(q) == 0 {
-		m.queues[variant] = append(q, accountID)
-		m.queued[accountID] = variant
+		m.queues[key] = append(q, accountID)
+		m.queued[accountID] = key
 		m.waitingSince[accountID] = m.clock()
 		m.mu.Unlock()
 		return EnqueueResult{}, nil
 	}
 	opponent := q[0]
-	m.removeLocked(opponent, variant)
+	m.removeLocked(opponent, key)
 	seats := []uuid.UUID{opponent, accountID}
 	if m.rng.Intn(2) == 0 {
 		seats[0], seats[1] = seats[1], seats[0]
 	}
 	m.mu.Unlock()
 
-	g, err := m.games.Create(ctx, autoMatchParams(variant, seats))
+	g, err := m.games.Create(ctx, autoMatchParams(key, seats))
 	if err != nil {
 		return EnqueueResult{}, err
 	}
@@ -161,19 +171,21 @@ func (m *Matchmaker) Cancel(_ context.Context, accountID uuid.UUID) bool {
 	m.mu.Lock()
 	defer m.mu.Unlock()
 	delete(m.results, accountID)
-	variant, ok := m.queued[accountID]
+	key, ok := m.queued[accountID]
 	if !ok {
 		return false
 	}
-	m.removeLocked(accountID, variant)
+	m.removeLocked(accountID, key)
 	return true
 }
 
-// QueueLen returns the number of accounts waiting in the variant pool.
+// QueueLen returns the number of accounts waiting in the variant pool, summed across
+// both per-turn word rules.
 func (m *Matchmaker) QueueLen(variant engine.Variant) int {
 	m.mu.Lock()
 	defer m.mu.Unlock()
-	return len(m.queues[variant])
+	return len(m.queues[matchKey{variant: variant, multipleWords: false}]) +
+		len(m.queues[matchKey{variant: variant, multipleWords: true}])
 }
 
 // RunReaper substitutes a robot for any player that has waited past waitDelay,
@@ -198,9 +210,9 @@ func (m *Matchmaker) RunReaper(ctx context.Context, interval time.Duration) {
 // momentarily empty pool just defers substitution to a later tick.
 func (m *Matchmaker) Reap(ctx context.Context, now time.Time) {
 	type sub struct {
-		human   uuid.UUID
-		variant engine.Variant
-		seats   []uuid.UUID
+		human uuid.UUID
+		key   matchKey
+		seats []uuid.UUID
 	}
 	m.mu.Lock()
 	var due []uuid.UUID
@@ -211,23 +223,23 @@ func (m *Matchmaker) Reap(ctx context.Context, now time.Time) {
 	}
 	var subs []sub
 	for _, acc := range due {
-		variant := m.queued[acc]
-		robotID, err := m.robots.Pick(variant)
+		key := m.queued[acc]
+		robotID, err := m.robots.Pick(key.variant)
 		if err != nil {
 			m.log.Warn("robot substitution deferred", zap.Error(err))
 			continue
 		}
-		m.removeLocked(acc, variant)
+		m.removeLocked(acc, key)
 		seats := []uuid.UUID{acc, robotID}
 		if m.rng.Intn(2) == 0 {
 			seats[0], seats[1] = seats[1], seats[0]
 		}
-		subs = append(subs, sub{human: acc, variant: variant, seats: seats})
+		subs = append(subs, sub{human: acc, key: key, seats: seats})
 	}
 	m.mu.Unlock()
 
 	for _, s := range subs {
-		g, err := m.games.Create(ctx, autoMatchParams(s.variant, s.seats))
+		g, err := m.games.Create(ctx, autoMatchParams(s.key, s.seats))
 		if err != nil {
 			m.log.Warn("robot substitution failed", zap.String("human", s.human.String()), zap.Error(err))
 			continue
@@ -241,13 +253,13 @@ func (m *Matchmaker) Reap(ctx context.Context, now time.Time) {
 
 // removeLocked drops accountID from the queue, the queued index and the waiting
 // clock. The caller holds m.mu.
-func (m *Matchmaker) removeLocked(accountID uuid.UUID, variant engine.Variant) {
+func (m *Matchmaker) removeLocked(accountID uuid.UUID, key matchKey) {
 	delete(m.queued, accountID)
 	delete(m.waitingSince, accountID)
-	q := m.queues[variant]
+	q := m.queues[key]
 	for i, id := range q {
 		if id == accountID {
-			m.queues[variant] = append(q[:i], q[i+1:]...)
+			m.queues[key] = append(q[:i], q[i+1:]...)
 			break
 		}
 	}
@@ -255,12 +267,13 @@ func (m *Matchmaker) removeLocked(accountID uuid.UUID, variant engine.Variant) {
 
 // autoMatchParams builds the create parameters for a two-player auto-match with
 // the casual defaults.
-func autoMatchParams(variant engine.Variant, seats []uuid.UUID) game.CreateParams {
+func autoMatchParams(key matchKey, seats []uuid.UUID) game.CreateParams {
 	return game.CreateParams{
-		Variant:        variant,
-		Seats:          seats,
-		TurnTimeout:    game.DefaultTurnTimeout,
-		HintsAllowed:   autoMatchHintsAllowed,
-		HintsPerPlayer: autoMatchHintsPerPlayer,
+		Variant:              key.variant,
+		Seats:                seats,
+		TurnTimeout:          game.DefaultTurnTimeout,
+		HintsAllowed:         autoMatchHintsAllowed,
+		HintsPerPlayer:       autoMatchHintsPerPlayer,
+		MultipleWordsPerTurn: key.multipleWords,
 	}
 }

backend/internal/lobby/matchmaker_test.go

@@ -80,7 +80,7 @@ func TestMatchmakerPairsTwoHumans(t *testing.T) {
 	ctx := context.Background()
 	a, b := uuid.New(), uuid.New()
 
-	r1, err := mm.Enqueue(ctx, a, engine.VariantEnglish)
+	r1, err := mm.Enqueue(ctx, a, engine.VariantEnglish, true)
 	if err != nil {
 		t.Fatalf("enqueue a: %v", err)
 	}
@@ -91,7 +91,7 @@ func TestMatchmakerPairsTwoHumans(t *testing.T) {
 		t.Fatalf("queue len = %d, want 1", mm.QueueLen(engine.VariantEnglish))
 	}
 
-	r2, err := mm.Enqueue(ctx, b, engine.VariantEnglish)
+	r2, err := mm.Enqueue(ctx, b, engine.VariantEnglish, true)
 	if err != nil {
 		t.Fatalf("enqueue b: %v", err)
 	}
@@ -129,10 +129,10 @@ func TestMatchmakerAlreadyQueued(t *testing.T) {
 	mm := newTestMatchmaker(&fakeCreator{}, uuid.New())
 	ctx := context.Background()
 	a := uuid.New()
-	if _, err := mm.Enqueue(ctx, a, engine.VariantEnglish); err != nil {
+	if _, err := mm.Enqueue(ctx, a, engine.VariantEnglish, true); err != nil {
 		t.Fatalf("enqueue: %v", err)
 	}
-	if _, err := mm.Enqueue(ctx, a, engine.VariantEnglish); !errors.Is(err, ErrAlreadyQueued) {
+	if _, err := mm.Enqueue(ctx, a, engine.VariantEnglish, true); !errors.Is(err, ErrAlreadyQueued) {
 		t.Fatalf("second enqueue err = %v, want ErrAlreadyQueued", err)
 	}
 }
@@ -141,7 +141,7 @@ func TestMatchmakerCancel(t *testing.T) {
 	mm := newTestMatchmaker(&fakeCreator{}, uuid.New())
 	ctx := context.Background()
 	a := uuid.New()
-	if _, err := mm.Enqueue(ctx, a, engine.VariantEnglish); err != nil {
+	if _, err := mm.Enqueue(ctx, a, engine.VariantEnglish, true); err != nil {
 		t.Fatalf("enqueue: %v", err)
 	}
 	if !mm.Cancel(ctx, a) {
@@ -159,10 +159,10 @@ func TestMatchmakerVariantsAreSeparate(t *testing.T) {
 	creator := &fakeCreator{}
 	mm := newTestMatchmaker(creator, uuid.New())
 	ctx := context.Background()
-	if _, err := mm.Enqueue(ctx, uuid.New(), engine.VariantEnglish); err != nil {
+	if _, err := mm.Enqueue(ctx, uuid.New(), engine.VariantEnglish, true); err != nil {
 		t.Fatalf("enqueue en: %v", err)
 	}
-	if _, err := mm.Enqueue(ctx, uuid.New(), engine.VariantRussianScrabble); err != nil {
+	if _, err := mm.Enqueue(ctx, uuid.New(), engine.VariantRussianScrabble, true); err != nil {
 		t.Fatalf("enqueue ru: %v", err)
 	}
 	if len(creator.created) != 0 {
@@ -179,7 +179,7 @@ func TestMatchmakerFIFO(t *testing.T) {
 	ctx := context.Background()
 	a, b, c := uuid.New(), uuid.New(), uuid.New()
 	for _, id := range []uuid.UUID{a, b, c} {
-		if _, err := mm.Enqueue(ctx, id, engine.VariantEnglish); err != nil {
+		if _, err := mm.Enqueue(ctx, id, engine.VariantEnglish, true); err != nil {
 			t.Fatalf("enqueue %s: %v", id, err)
 		}
 	}
@@ -204,7 +204,7 @@ func TestMatchmakerReaperSubstitutesRobot(t *testing.T) {
 	ctx := context.Background()
 	a := uuid.New()
 
-	if _, err := mm.Enqueue(ctx, a, engine.VariantEnglish); err != nil {
+	if _, err := mm.Enqueue(ctx, a, engine.VariantEnglish, true); err != nil {
 		t.Fatalf("enqueue: %v", err)
 	}
 
@@ -237,7 +237,7 @@ func TestMatchmakerReaperSkipsCancelled(t *testing.T) {
 	ctx := context.Background()
 	a := uuid.New()
 
-	if _, err := mm.Enqueue(ctx, a, engine.VariantEnglish); err != nil {
+	if _, err := mm.Enqueue(ctx, a, engine.VariantEnglish, true); err != nil {
 		t.Fatalf("enqueue: %v", err)
 	}
 	mm.Cancel(ctx, a)
@@ -258,7 +258,7 @@ func TestMatchmakerCancelClearsPendingResult(t *testing.T) {
 	ctx := context.Background()
 	a := uuid.New()
 
-	if _, err := mm.Enqueue(ctx, a, engine.VariantEnglish); err != nil {
+	if _, err := mm.Enqueue(ctx, a, engine.VariantEnglish, true); err != nil {
 		t.Fatalf("enqueue: %v", err)
 	}
 	mm.Reap(ctx, base.Add(testWaitDelay+time.Second)) // substitution stores a pending result
@@ -276,7 +276,7 @@ func TestMatchmakerReaperDefersWithoutRobot(t *testing.T) {
 	ctx := context.Background()
 	a := uuid.New()
 
-	if _, err := mm.Enqueue(ctx, a, engine.VariantEnglish); err != nil {
+	if _, err := mm.Enqueue(ctx, a, engine.VariantEnglish, true); err != nil {
 		t.Fatalf("enqueue: %v", err)
 	}
 	mm.Reap(ctx, base.Add(testWaitDelay+time.Second))
@@ -287,3 +287,57 @@ func TestMatchmakerReaperDefersWithoutRobot(t *testing.T) {
 		t.Errorf("waiter must stay queued when substitution is deferred; len %d", mm.QueueLen(engine.VariantEnglish))
 	}
 }
+
+// TestMatchmakerRulesAreSeparate confirms two players who chose the same variant but a
+// different per-turn word rule are not paired, and that the rule reaches the started game.
+func TestMatchmakerRulesAreSeparate(t *testing.T) {
+	creator := &fakeCreator{}
+	mm := newTestMatchmaker(creator, uuid.New())
+	ctx := context.Background()
+
+	// Same variant, opposite rules: they must not match.
+	if _, err := mm.Enqueue(ctx, uuid.New(), engine.VariantRussianScrabble, false); err != nil {
+		t.Fatalf("enqueue single-word: %v", err)
+	}
+	if _, err := mm.Enqueue(ctx, uuid.New(), engine.VariantRussianScrabble, true); err != nil {
+		t.Fatalf("enqueue standard: %v", err)
+	}
+	if len(creator.created) != 0 {
+		t.Fatalf("different rules must not match; created %d", len(creator.created))
+	}
+
+	// A second single-word player pairs with the first; the game carries the rule.
+	r, err := mm.Enqueue(ctx, uuid.New(), engine.VariantRussianScrabble, false)
+	if err != nil {
+		t.Fatalf("enqueue single-word opponent: %v", err)
+	}
+	if !r.Matched {
+		t.Fatal("same variant and rule must match")
+	}
+	if len(creator.created) != 1 {
+		t.Fatalf("created %d games, want 1", len(creator.created))
+	}
+	if creator.created[0].MultipleWordsPerTurn {
+		t.Error("single-word match must create a game with MultipleWordsPerTurn=false")
+	}
+}
+
+// TestMatchmakerReaperKeepsRule confirms a robot substitution carries the waiter's rule.
+func TestMatchmakerReaperKeepsRule(t *testing.T) {
+	creator := &fakeCreator{}
+	mm := newTestMatchmaker(creator, uuid.New())
+	base := time.Now()
+	mm.clock = func() time.Time { return base }
+	ctx := context.Background()
+
+	if _, err := mm.Enqueue(ctx, uuid.New(), engine.VariantRussianScrabble, false); err != nil {
+		t.Fatalf("enqueue: %v", err)
+	}
+	mm.Reap(ctx, base.Add(testWaitDelay+time.Second))
+	if len(creator.created) != 1 {
+		t.Fatalf("created %d games, want 1", len(creator.created))
+	}
+	if creator.created[0].MultipleWordsPerTurn {
+		t.Error("robot substitution must keep the waiter's single-word rule")
+	}
+}