feat: Implement texture caching system with dirty flag optimization (closes #144)

- Add cache_subtree property on Frame for opt-in RenderTexture caching
- Add PyTexture::from_rendered() factory for runtime texture creation
- Add snapshot= parameter to Sprite for creating sprites from Frame content
- Implement content_dirty vs composite_dirty distinction:
  - markContentDirty(): content changed, invalidate self and ancestors
  - markCompositeDirty(): position changed, ancestors need recomposite only
- Update all UIDrawable position setters to use markCompositeDirty()
- Add quick exit workaround for cleanup segfaults

Benchmark: deep_nesting_cached is 3.7x faster (0.09ms vs 0.35ms)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

src/GameEngine.cpp

@@ -338,9 +338,14 @@ void GameEngine::run()
             running = false;
         }
     }
-    
+
     // Clean up before exiting the run loop
     cleanup();
+
+    // #144: Quick exit to avoid cleanup segfaults in Python/C++ destructor ordering
+    // This is a pragmatic workaround - proper cleanup would require careful
+    // attention to shared_ptr cycles and Python GC interaction
+    std::_Exit(0);
 }
 
 std::shared_ptr<Timer> GameEngine::getTimer(const std::string& name)