feat(display): add Kitty graphics backend and improve font detection

- Add KittyDisplay using kitty's native graphics protocol
- Improve cross-platform font detection for SixelDisplay
- Add run-kitty mise task for testing kitty backend
- Add kitty_test.py for testing graphics protocol

engine/display/__init__.py

@@ -7,6 +7,7 @@ Supports auto-discovery of display backends.
 
 from typing import Protocol
 
+from engine.display.backends.kitty import KittyDisplay
 from engine.display.backends.multi import MultiDisplay
 from engine.display.backends.null import NullDisplay
 from engine.display.backends.sixel import SixelDisplay
@@ -76,6 +77,7 @@ class DisplayRegistry:
         cls.register("null", NullDisplay)
         cls.register("websocket", WebSocketDisplay)
         cls.register("sixel", SixelDisplay)
+        cls.register("kitty", KittyDisplay)
 
         cls._initialized = True