docs: update development state with cursor movement fix

The split window functionality is now complete with proper cursor movement.
Window splits work correctly with:
- Proper window focus synchronization
- Cursor movement in the correct focused window
- Text input targeting the right window
- Stable operation without crashes or freezes

Only remaining minor issue is uneven split ratios, which can be addressed
in a future optimization phase.

DEV_STATE.md

@@ -71,11 +71,12 @@ Lumacs/
 - ✅ **Multi-Window Text Rendering**: Fixed text rendering in split windows with proper font metrics
 - ✅ **Split Window UI Polish**: Fixed modeline display, focus stability, and split ratio calculations
 - ✅ **Window Split Freeze Fix**: Resolved GTK signal_realize callback issues causing freezes during splits
+- ✅ **Split Window Cursor Fix**: Fixed cursor movement to work in focused window rather than original active window
 
 ## Todo  
 1. **Phase 7**: Performance optimization and tuning
 2. **Phase 8**: Mouse support and advanced UI features
-3. **Window Focusing API**: Add proper `set_active_window` method to EditorCore instead of cycling through next_window
+3. **Split Ratio Improvement**: Implement proper 50/50 split ratios without GTK signal issues
 
 ## Technical Debt/Notes
 - **Lua Bridge**: The lua_api.cpp contains the critical C++/Lua boundary code