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.

gateway/internal/backendclient/api_social.go

@@ -99,6 +99,8 @@ type InvitationParams struct {
 	HintsAllowed    bool
 	HintsPerPlayer  int
 	DropoutTiles    string
+	// MultipleWordsPerTurn true is standard Scrabble; false the single-word rule.
+	MultipleWordsPerTurn bool
 }
 
 // --- friends ---
@@ -195,6 +197,8 @@ func (c *Client) CreateInvitation(ctx context.Context, userID string, p Invitati
 		"hints_allowed":     p.HintsAllowed,
 		"hints_per_player":  p.HintsPerPlayer,
 		"dropout_tiles":     p.DropoutTiles,
+
+		"multiple_words_per_turn": p.MultipleWordsPerTurn,
 	}
 	err := c.do(ctx, http.MethodPost, "/api/v1/user/invitations", userID, "", body, &out)
 	return out, err