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.

pkg/fbs/scrabble.fbs

@@ -272,9 +272,11 @@ table GameList {
 
 // --- lobby (authenticated) ---
 
-// EnqueueRequest joins the per-variant auto-match pool.
+// EnqueueRequest joins the auto-match pool for a variant under a per-turn word rule.
+// multiple_words_per_turn true is standard Scrabble; false is the single-word rule.
 table EnqueueRequest {
   variant:string;
+  multiple_words_per_turn:bool;
 }
 
 // MatchResult reports whether the caller has been paired into a game yet.
@@ -457,6 +459,7 @@ table CreateInvitationRequest {
   hints_allowed:bool;
   hints_per_player:int;
   dropout_tiles:string;
+  multiple_words_per_turn:bool;
 }
 
 // InvitationActionRequest accepts / declines / cancels an invitation by id.