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.

ui/src/lib/variants.test.ts

@@ -1,6 +1,11 @@
 import { describe, it, expect } from 'vitest';
 
-import { ALL_VARIANTS, availableVariants } from './variants';
+import {
+  ALL_VARIANTS,
+  availableVariants,
+  supportsMultipleWordsToggle,
+  multipleWordsForRequest,
+} from './variants';
 
 describe('availableVariants', () => {
   it('is ungated (all variants) for an empty or absent set', () => {
@@ -20,3 +25,24 @@ describe('availableVariants', () => {
     expect(availableVariants(['en', 'ru']).map((v) => v.id)).toEqual(['scrabble_en', 'scrabble_ru', 'erudit_ru']);
   });
 });
+
+describe('supportsMultipleWordsToggle', () => {
+  it('is true for Russian variants only', () => {
+    expect(supportsMultipleWordsToggle('scrabble_ru')).toBe(true);
+    expect(supportsMultipleWordsToggle('erudit_ru')).toBe(true);
+    expect(supportsMultipleWordsToggle('scrabble_en')).toBe(false);
+  });
+});
+
+describe('multipleWordsForRequest', () => {
+  it('carries the toggle for Russian games', () => {
+    expect(multipleWordsForRequest('scrabble_ru', false)).toBe(false);
+    expect(multipleWordsForRequest('scrabble_ru', true)).toBe(true);
+    expect(multipleWordsForRequest('erudit_ru', false)).toBe(false);
+  });
+
+  it('forces standard (true) for English whatever the toggle', () => {
+    expect(multipleWordsForRequest('scrabble_en', false)).toBe(true);
+    expect(multipleWordsForRequest('scrabble_en', true)).toBe(true);
+  });
+});