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.ts

@@ -55,3 +55,17 @@ export function availableVariants(supportedLanguages: string[] | undefined): Var
   if (langs.length === 0) return ALL_VARIANTS;
   return ALL_VARIANTS.filter((v) => langs.includes(VARIANT_LANGUAGE[v.id]));
 }
+
+// supportsMultipleWordsToggle reports whether the New Game "multiple words per turn" toggle
+// applies to a variant. Only Russian games choose the rule; English is always standard, so
+// its toggle is not shown.
+export function supportsMultipleWordsToggle(v: Variant): boolean {
+  return VARIANT_LANGUAGE[v] === 'ru';
+}
+
+// multipleWordsForRequest resolves the per-turn word rule sent when starting a game of the
+// variant: Russian games carry the toggle's value, English games are silently standard
+// (true), so matchmaking and game creation stay one uniform path.
+export function multipleWordsForRequest(v: Variant, toggle: boolean): boolean {
+  return supportsMultipleWordsToggle(v) ? toggle : true;
+}