BSP: add Binary Space Partitioning for procedural dungeon generation

Implements #202, #203, #204, #205; partially implements #206:
- BSP class: core tree structure with bounds, split_once, split_recursive, clear
- BSPNode class: lightweight node reference with bounds, level, is_leaf,
  split_horizontal, split_position; navigation via left/right/parent/sibling;
  contains() and center() methods
- Traversal enum: PRE_ORDER, IN_ORDER, POST_ORDER, LEVEL_ORDER, INVERTED_LEVEL_ORDER
- BSP iteration: leaves() for leaf nodes only, traverse(order) for all nodes
- BSP query: find(pos) returns deepest node containing position
- BSP.to_heightmap(): converts BSP to HeightMap with select, shrink, value options

Note: #206's BSPMap subclass deferred - to_heightmap returns plain HeightMap.
The HeightMap already has all necessary operations (inverse, threshold, etc.)
for procedural generation workflows.

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

tests/unit/bsp_simple_test.py

@@ -0,0 +1,28 @@
+"""Simple BSP test to identify crash."""
+import sys
+import mcrfpy
+
+print("Step 1: Import complete")
+
+print("Step 2: Creating BSP...")
+bsp = mcrfpy.BSP(bounds=((0, 0), (100, 80)))
+print("Step 2: BSP created:", bsp)
+
+print("Step 3: Getting bounds...")
+bounds = bsp.bounds
+print("Step 3: Bounds:", bounds)
+
+print("Step 4: Getting root...")
+root = bsp.root
+print("Step 4: Root:", root)
+
+print("Step 5: Split once...")
+bsp.split_once(horizontal=True, position=40)
+print("Step 5: Split complete")
+
+print("Step 6: Get leaves...")
+leaves = list(bsp.leaves())
+print("Step 6: Leaves count:", len(leaves))
+
+print("PASS")
+sys.exit(0)