feat: migrate web UI to Cython-optimized game engine

- Add cpdef wrappers (get_placed_squares, get_placed_corners) for Python
  accessibility of previously cdef-only methods
- Add get_standard_pieces() to expose Cython standard piece set
- Add is_game_over(), get_scores(), get_winners() to Cython BlokusGame
- Fix valid_move() player_occupied lookup (was using player_idx instead
  of player_idx+1, causing bots to fail after first move)
- Update session.py, app.py, bots.py to use Cython with pure Python fallback
- Fix board.grid.tolist() to np.array(grid).tolist() for memoryview compat
- Add Cython build artifacts to .gitignore

.gitignore

@@ -6,3 +6,8 @@ __pycache__/
 .ruff_cache/
 *.egg-info/
 runs/
+# Cython build artifacts
+*.so
+*.o
+blokus_cython.c
+build/