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/scrabblefb/EnqueueRequest.go

@@ -49,12 +49,27 @@ func (rcv *EnqueueRequest) Variant() []byte {
 	return nil
 }
 
+func (rcv *EnqueueRequest) MultipleWordsPerTurn() bool {
+	o := flatbuffers.UOffsetT(rcv._tab.Offset(6))
+	if o != 0 {
+		return rcv._tab.GetBool(o + rcv._tab.Pos)
+	}
+	return false
+}
+
+func (rcv *EnqueueRequest) MutateMultipleWordsPerTurn(n bool) bool {
+	return rcv._tab.MutateBoolSlot(6, n)
+}
+
 func EnqueueRequestStart(builder *flatbuffers.Builder) {
-	builder.StartObject(1)
+	builder.StartObject(2)
 }
 func EnqueueRequestAddVariant(builder *flatbuffers.Builder, variant flatbuffers.UOffsetT) {
 	builder.PrependUOffsetTSlot(0, flatbuffers.UOffsetT(variant), 0)
 }
+func EnqueueRequestAddMultipleWordsPerTurn(builder *flatbuffers.Builder, multipleWordsPerTurn bool) {
+	builder.PrependBoolSlot(1, multipleWordsPerTurn, false)
+}
 func EnqueueRequestEnd(builder *flatbuffers.Builder) flatbuffers.UOffsetT {
 	return builder.EndObject()
 }