Add pure-Dart engine core: RNG, grid, placement, line clear, scoring, piece generator

PCG32 seeded RNG; immutable 8x8 GridState with occupancy bitmask;
placement legality + anyPlacementExists; simultaneous row/col clears
with single-count gem credit; combo scoring with one-move grace;
weighted-bag generator with pity bias and depth-3 solvability nudge.
All TDD, 51 tests green.

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

lib/game/models/piece.dart

@@ -0,0 +1,27 @@
+/// A polyomino with fixed orientation (no rotation at play time, genre
+/// convention). Offsets are normalized cell positions relative to the
+/// top-left anchor.
+class Piece {
+  const Piece({
+    required this.id,
+    required this.offsets,
+    required this.colorId,
+    this.weight = 1.0,
+    this.tier = 1,
+  });
+
+  final String id;
+  final List<(int, int)> offsets;
+  final int colorId;
+
+  /// Base draw weight; stage generator profiles scale this.
+  final double weight;
+
+  /// Difficulty tier 1 (easy to fit) .. 3 (demanding).
+  final int tier;
+
+  int get size => offsets.length;
+
+  @override
+  String toString() => 'Piece($id)';
+}