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.

docs/ARCHITECTURE.md

@@ -282,6 +282,17 @@ Key points:
 
 - **Word legality: validate-at-submit.** An illegal play is rejected by
   `Solver.ValidatePlay`; there is no challenge phase.
+- **Multiple words per turn (Russian games).** Russian variants carry a per-game
+  **single-word rule**, chosen on New Game (default **off** = single word; 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; on, every
+  cross-word must be a real word and is scored. The engine threads it as
+  `scrabble.PlayOptions{IgnoreCrossWords}` (solver `v1.1.0`); connectivity and the
+  first-move centre rule are unaffected. The "Russian-only" limit is a **UI affordance**:
+  the backend and engine are variant-agnostic about the flag, and English games always send
+  it on (standard). For auto-match the rule is part of the matchmaking key, so only players
+  who chose the same rule are paired (the rule field rides every create/enqueue request, so
+  matchmaking stays one uniform path).
 - **End of game**: the bag is empty **and** a player empties their rack, **or**
   **6 consecutive scoreless turns** (passes/exchanges), **or** a resignation, or
   a missed turn. The **per-game turn timeout** is chosen at creation