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.

backend/internal/engine/singleword_test.go

@@ -0,0 +1,48 @@
+package engine
+
+import "testing"
+
+// TestSingleWordRuleWiring confirms Options.MultipleWordsPerTurn reaches the solver. The
+// single-word game ignores perpendicular cross-words, so move generation from a shared
+// position is a superset of the standard game's; the standard game does not relax them.
+func TestSingleWordRuleWiring(t *testing.T) {
+	const seed = 7
+	mk := func(multipleWords bool) *Game {
+		g, err := New(testReg, Options{
+			Variant:              VariantEnglish,
+			Version:              testVersion,
+			Players:              2,
+			Seed:                 seed,
+			MultipleWordsPerTurn: multipleWords,
+		})
+		if err != nil {
+			t.Fatalf("new game: %v", err)
+		}
+		return g
+	}
+	std, single := mk(true), mk(false)
+	if std.playOpts().IgnoreCrossWords {
+		t.Error("standard game must not ignore cross-words")
+	}
+	if !single.playOpts().IgnoreCrossWords {
+		t.Error("single-word game must ignore cross-words")
+	}
+
+	// Play the same opening (the standard game's top move) in both games, then compare
+	// the next player's candidate moves. Both games share the seed, so the next rack is
+	// identical; relaxed (single-word) generation never drops a legal standard move.
+	hint, ok := std.HintView()
+	if !ok {
+		t.Fatal("opening game has no hint")
+	}
+	if _, err := std.SubmitPlay(hint.Tiles); err != nil {
+		t.Fatalf("standard opening: %v", err)
+	}
+	if _, err := single.SubmitPlay(hint.Tiles); err != nil {
+		t.Fatalf("single-word opening: %v", err)
+	}
+	stdMoves, singleMoves := len(std.GenerateMoves()), len(single.GenerateMoves())
+	if singleMoves < stdMoves {
+		t.Errorf("single-word generation produced %d moves, want >= standard %d", singleMoves, stdMoves)
+	}
+}