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/models/stage.dart

@@ -74,8 +74,21 @@ class StageConfig {
     required this.objectives,
     required this.stars,
     required this.generatorProfile,
+    this.endless = false,
   });
 
+  factory StageConfig.endless({required int seed}) => StageConfig(
+        id: 'endless',
+        seed: seed,
+        moveLimit: 0,
+        preset: const [],
+        objectives: const [],
+        stars: const StarThresholds(
+            twoMovesLeft: 1 << 30, threeMovesLeft: 1 << 30),
+        generatorProfile: 'mid',
+        endless: true,
+      );
+
   factory StageConfig.fromJson(Map<String, dynamic> json) => StageConfig(
         id: json['id'] as String,
         seed: json['seed'] as int,
@@ -100,6 +113,10 @@ class StageConfig {
   final StarThresholds stars;
   final String generatorProfile;
 
+  /// Runtime-only: score-attack mode with no objectives or move limit.
+  /// Never serialized — packs always describe objective stages.
+  final bool endless;
+
   GridState initialGrid() {
     var grid = GridState.empty();
     for (final cell in preset) {