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/gen/fbs/scrabblefb/create-invitation-request.ts

@@ -61,8 +61,13 @@ dropoutTiles(optionalEncoding?:any):string|Uint8Array|null {
   return offset ? this.bb!.__string(this.bb_pos + offset, optionalEncoding) : null;
 }
 
+multipleWordsPerTurn():boolean {
+  const offset = this.bb!.__offset(this.bb_pos, 16);
+  return offset ? !!this.bb!.readInt8(this.bb_pos + offset) : false;
+}
+
 static startCreateInvitationRequest(builder:flatbuffers.Builder) {
-  builder.startObject(6);
+  builder.startObject(7);
 }
 
 static addInviteeIds(builder:flatbuffers.Builder, inviteeIdsOffset:flatbuffers.Offset) {
@@ -101,12 +106,16 @@ static addDropoutTiles(builder:flatbuffers.Builder, dropoutTilesOffset:flatbuffe
   builder.addFieldOffset(5, dropoutTilesOffset, 0);
 }
 
+static addMultipleWordsPerTurn(builder:flatbuffers.Builder, multipleWordsPerTurn:boolean) {
+  builder.addFieldInt8(6, +multipleWordsPerTurn, +false);
+}
+
 static endCreateInvitationRequest(builder:flatbuffers.Builder):flatbuffers.Offset {
   const offset = builder.endObject();
   return offset;
 }
 
-static createCreateInvitationRequest(builder:flatbuffers.Builder, inviteeIdsOffset:flatbuffers.Offset, variantOffset:flatbuffers.Offset, turnTimeoutSecs:number, hintsAllowed:boolean, hintsPerPlayer:number, dropoutTilesOffset:flatbuffers.Offset):flatbuffers.Offset {
+static createCreateInvitationRequest(builder:flatbuffers.Builder, inviteeIdsOffset:flatbuffers.Offset, variantOffset:flatbuffers.Offset, turnTimeoutSecs:number, hintsAllowed:boolean, hintsPerPlayer:number, dropoutTilesOffset:flatbuffers.Offset, multipleWordsPerTurn:boolean):flatbuffers.Offset {
   CreateInvitationRequest.startCreateInvitationRequest(builder);
   CreateInvitationRequest.addInviteeIds(builder, inviteeIdsOffset);
   CreateInvitationRequest.addVariant(builder, variantOffset);
@@ -114,6 +123,7 @@ static createCreateInvitationRequest(builder:flatbuffers.Builder, inviteeIdsOffs
   CreateInvitationRequest.addHintsAllowed(builder, hintsAllowed);
   CreateInvitationRequest.addHintsPerPlayer(builder, hintsPerPlayer);
   CreateInvitationRequest.addDropoutTiles(builder, dropoutTilesOffset);
+  CreateInvitationRequest.addMultipleWordsPerTurn(builder, multipleWordsPerTurn);
   return CreateInvitationRequest.endCreateInvitationRequest(builder);
 }
 }

ui/src/gen/fbs/scrabblefb/enqueue-request.ts

@@ -27,22 +27,32 @@ variant(optionalEncoding?:any):string|Uint8Array|null {
   return offset ? this.bb!.__string(this.bb_pos + offset, optionalEncoding) : null;
 }
 
+multipleWordsPerTurn():boolean {
+  const offset = this.bb!.__offset(this.bb_pos, 6);
+  return offset ? !!this.bb!.readInt8(this.bb_pos + offset) : false;
+}
+
 static startEnqueueRequest(builder:flatbuffers.Builder) {
-  builder.startObject(1);
+  builder.startObject(2);
 }
 
 static addVariant(builder:flatbuffers.Builder, variantOffset:flatbuffers.Offset) {
   builder.addFieldOffset(0, variantOffset, 0);
 }
 
+static addMultipleWordsPerTurn(builder:flatbuffers.Builder, multipleWordsPerTurn:boolean) {
+  builder.addFieldInt8(1, +multipleWordsPerTurn, +false);
+}
+
 static endEnqueueRequest(builder:flatbuffers.Builder):flatbuffers.Offset {
   const offset = builder.endObject();
   return offset;
 }
 
-static createEnqueueRequest(builder:flatbuffers.Builder, variantOffset:flatbuffers.Offset):flatbuffers.Offset {
+static createEnqueueRequest(builder:flatbuffers.Builder, variantOffset:flatbuffers.Offset, multipleWordsPerTurn:boolean):flatbuffers.Offset {
   EnqueueRequest.startEnqueueRequest(builder);
   EnqueueRequest.addVariant(builder, variantOffset);
+  EnqueueRequest.addMultipleWordsPerTurn(builder, multipleWordsPerTurn);
   return EnqueueRequest.endEnqueueRequest(builder);
 }
 }