Add objectives, stage config, and GameEngine session core

Sealed Objective types (clearGems/reachScore/clearLines) with JSON
round-trip; StageConfig with preset cells and star thresholds;
GameEngine orchestrating placement -> clear -> scoring -> objectives
with stuck detection, one-shot rescue (continue / +5 moves), and
deterministic per-attempt RNG. 100-game headless stress test and
pure-Dart architecture guard. 76 tests green.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

lib/game/engine/piece_generator.dart

@@ -97,7 +97,7 @@ class PieceGenerator {
         break;
       }
     }
-    final tray = <Piece>[if (largestFitting != null) largestFitting];
+    final tray = <Piece>[?largestFitting];
     for (final p in bySizeDesc.reversed) {
       if (tray.length == traySize) break;
       if (tray.any((t) => t.id == p.id)) continue;