feat: Thread-safe FOV system with improved API

Major improvements to the Field of View (FOV) system:

1. Added thread safety with mutex protection
   - Added mutable std::mutex fov_mutex to UIGrid class
   - Protected computeFOV() and isInFOV() with lock_guard
   - Minimal overhead for current single-threaded operation
   - Ready for future multi-threading requirements

2. Enhanced compute_fov() API to return visible cells
   - Changed return type from void to List[Tuple[int, int, bool, bool]]
   - Returns (x, y, visible, discovered) for all visible cells
   - Maintains backward compatibility by still updating internal FOV state
   - Allows FOV queries without affecting entity states

3. Fixed Part 4 tutorial visibility rendering
   - Added required entity.update_visibility() calls after compute_fov()
   - Fixed black grid issue in perspective rendering
   - Updated hallway generation to use L-shaped corridors

The architecture now properly separates concerns while maintaining
performance and preparing for future enhancements. Each entity can
have independent FOV calculations without race conditions.

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

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

src/UIGrid.h

@@ -6,6 +6,7 @@
 #include "Resources.h"
 #include <list>
 #include <libtcod.h>
+#include <mutex>
 
 #include "PyCallable.h"
 #include "PyTexture.h"
@@ -29,6 +30,7 @@ private:
     TCODMap* tcod_map;  // TCOD map for FOV and pathfinding
     TCODDijkstra* tcod_dijkstra;  // Dijkstra pathfinding
     TCODPath* tcod_path;  // A* pathfinding
+    mutable std::mutex fov_mutex;  // Mutex for thread-safe FOV operations
     
 public:
     UIGrid();