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/cell.dart

@@ -0,0 +1,22 @@
+enum CellType { empty, filled, gem }
+
+class Cell {
+  const Cell(this.type, {this.colorId = 0});
+
+  final CellType type;
+  final int colorId;
+
+  static const empty = Cell(CellType.empty);
+
+  bool get isOccupied => type != CellType.empty;
+
+  @override
+  bool operator ==(Object other) =>
+      other is Cell && other.type == type && other.colorId == colorId;
+
+  @override
+  int get hashCode => Object.hash(type, colorId);
+
+  @override
+  String toString() => 'Cell(${type.name}, color: $colorId)';
+}