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>

ios/Podfile.lock

@@ -0,0 +1,37 @@
+PODS:
+  - audioplayers_darwin (0.0.1):
+    - Flutter
+    - FlutterMacOS
+  - Flutter (1.0.0)
+  - path_provider_foundation (0.0.1):
+    - Flutter
+    - FlutterMacOS
+  - shared_preferences_foundation (0.0.1):
+    - Flutter
+    - FlutterMacOS
+
+DEPENDENCIES:
+  - audioplayers_darwin (from `.symlinks/plugins/audioplayers_darwin/darwin`)
+  - Flutter (from `Flutter`)
+  - path_provider_foundation (from `.symlinks/plugins/path_provider_foundation/darwin`)
+  - shared_preferences_foundation (from `.symlinks/plugins/shared_preferences_foundation/darwin`)
+
+EXTERNAL SOURCES:
+  audioplayers_darwin:
+    :path: ".symlinks/plugins/audioplayers_darwin/darwin"
+  Flutter:
+    :path: Flutter
+  path_provider_foundation:
+    :path: ".symlinks/plugins/path_provider_foundation/darwin"
+  shared_preferences_foundation:
+    :path: ".symlinks/plugins/shared_preferences_foundation/darwin"
+
+SPEC CHECKSUMS:
+  audioplayers_darwin: 835ced6edd4c9fc8ebb0a7cc9e294a91d99917d5
+  Flutter: cabc95a1d2626b1b06e7179b784ebcf0c0cde467
+  path_provider_foundation: bb55f6dbba17d0dccd6737fe6f7f34fbd0376880
+  shared_preferences_foundation: 7036424c3d8ec98dfe75ff1667cb0cd531ec82bb
+
+PODFILE CHECKSUM: 3c63482e143d1b91d2d2560aee9fb04ecc74ac7e
+
+COCOAPODS: 1.16.2