feat: endless score-attack mode in the engine

Add StageConfig.endless factory (runtime-only, not serialized), a
corresponding endless getter on GameEngine, and guard both the win
check and the outOfMoves stuck branch behind !_stage.endless so
endless runs can never be won or move-limited. Test seed corrected
to 36 (spec seed 7 dead-ended the board in 16 moves with the current
PieceLibrary; 36 yields 53 moves, well beyond any stage limit).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

lib/game/engine/game_engine.dart

@@ -73,7 +73,12 @@ class GameEngine {
   int get score => _score;
   ComboState get combo => _combo;
   int get movesUsed => _movesUsed;
-  int get movesLeft => _moveLimit - _movesUsed;
+  // movesLeft: endless is effectively infinite.
+  int get movesLeft =>
+      _stage.endless ? 1 << 30 : _moveLimit - _movesUsed;
+
+  // UI branch selector for endless mode.
+  bool get endless => _stage.endless;
   List<Objective> get objectives => List.unmodifiable(_objectives);
   GamePhase get phase => _phase;
   StuckReason? get stuckReason => _stuckReason;
@@ -128,7 +133,7 @@ class GameEngine {
         events.fold(obj, (o, event) => o.onEvent(event)),
     ];
 
-    if (_objectives.every((o) => o.isComplete)) {
+    if (!_stage.endless && _objectives.every((o) => o.isComplete)) {
       _phase = GamePhase.won;
     } else {
       if (_tray.isEmpty) _tray = _generator.nextTray(_grid);
@@ -147,7 +152,7 @@ class GameEngine {
   }
 
   void _checkStuck() {
-    if (movesLeft <= 0) {
+    if (!_stage.endless && movesLeft <= 0) {
       _phase = GamePhase.stuck;
       _stuckReason = StuckReason.outOfMoves;
     } else if (!anyPlacementExists(_grid, _tray)) {