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/codec.ts

@@ -149,11 +149,12 @@ export function encodeComplaint(gameId: string, word: string, note: string): Uin
   return finish(b, fb.ComplaintRequest.endComplaintRequest(b));
 }
 
-export function encodeEnqueue(variant: Variant): Uint8Array {
+export function encodeEnqueue(variant: Variant, multipleWords: boolean): Uint8Array {
   const b = new Builder(64);
   const v = b.createString(variant);
   fb.EnqueueRequest.startEnqueueRequest(b);
   fb.EnqueueRequest.addVariant(b, v);
+  fb.EnqueueRequest.addMultipleWordsPerTurn(b, multipleWords);
   return finish(b, fb.EnqueueRequest.endEnqueueRequest(b));
 }
 
@@ -523,6 +524,7 @@ export function encodeCreateInvitation(inviteeIds: string[], st: InvitationSetti
   fb.CreateInvitationRequest.addHintsAllowed(b, st.hintsAllowed);
   fb.CreateInvitationRequest.addHintsPerPlayer(b, st.hintsPerPlayer);
   fb.CreateInvitationRequest.addDropoutTiles(b, dropout);
+  fb.CreateInvitationRequest.addMultipleWordsPerTurn(b, st.multipleWordsPerTurn);
   return finish(b, fb.CreateInvitationRequest.endCreateInvitationRequest(b));
 }