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/Runner.xcworkspace/contents.xcworkspacedata

@@ -4,4 +4,7 @@
    <FileRef
       location = "group:Runner.xcodeproj">
    </FileRef>
+   <FileRef
+      location = "group:Pods/Pods.xcodeproj">
+   </FileRef>
 </Workspace>