BSP: add safety features and API improvements (closes #202, #203, #204, #205, #206)

Safety improvements:
- Generation counter detects stale BSPNode references after clear()/split_recursive()
- GRID_MAX validation prevents oversized BSP trees
- Depth parameter capped at 16 to prevent resource exhaustion
- Iterator checks generation to detect invalidation during mutation

API improvements:
- Changed constructor from bounds=((x,y),(w,h)) to pos=(x,y), size=(w,h)
- Added pos and size properties alongside bounds
- BSPNode __eq__ compares underlying pointers for identity
- BSP __iter__ as shorthand for leaves()
- BSP __len__ returns leaf count

Tests:
- Added tests for stale node detection, GRID_MAX validation, depth cap
- Added tests for __len__, __iter__, and BSPNode equality

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

tests/unit/bsp_simple_test.py

@@ -5,7 +5,7 @@ import mcrfpy
 print("Step 1: Import complete")
 
 print("Step 2: Creating BSP...")
-bsp = mcrfpy.BSP(bounds=((0, 0), (100, 80)))
+bsp = mcrfpy.BSP(pos=(0, 0), size=(100, 80))
 print("Step 2: BSP created:", bsp)
 
 print("Step 3: Getting bounds...")