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/e2e/game.spec.ts

@@ -70,6 +70,18 @@ test('new game: variant buttons show a rules summary and the move-limit', async
   await expect(page.locator('.movelimit')).toBeVisible(); // turn-time under the buttons
 });
 
+test('new game: Russian games offer the "multiple words per turn" toggle, off by default', async ({ page }) => {
+  await page.goto('/');
+  await page.getByRole('button', { name: /guest/i }).click();
+  await page.getByRole('button', { name: /New/ }).click(); // auto-match
+  // The mock session supports Russian, so the single-word-rule toggle is offered and starts off.
+  const toggle = page.getByLabel('Multiple words per turn');
+  await expect(toggle).toBeVisible();
+  await expect(toggle).not.toBeChecked();
+  await toggle.check();
+  await expect(toggle).toBeChecked();
+});
+
 test('a pending tile recalls on double-tap, not on a single tap', async ({ page }) => {
   await openGame(page);
   await page.locator('.rack .tile').first().click();