refactor(display)!: remove deprecated backends, simplify protocol, and add BorderMode/UI rendering

- Remove SixelDisplay and KittyDisplay backends (unmaintained)
- Simplify Display protocol: reduce docstring noise, emphasize duck typing
- Add BorderMode enum (OFF, SIMPLE, UI) for flexible border rendering
- Rename render_border to _render_simple_border
- Add render_ui_panel() to compose main viewport with right-side UI panel
- Add new render_border() dispatcher supporting BorderMode
- Update __all__ to expose BorderMode, render_ui_panel, PygameDisplay
- Clean up DisplayRegistry: remove deprecated method docstrings
- Update tests: remove SixelDisplay import, assert sixel not in registry
- Add TODO comment to WebSocket backend about streaming improvements

This is a breaking change (removal of backends) but enables cleaner architecture and interactive UI panel.

Closes #13, #21

engine/display/backends/websocket.py

@@ -1,5 +1,14 @@
 """
 WebSocket display backend - broadcasts frame buffer to connected web clients.
+
+TODO: Transform to a true streaming backend with:
+- Proper WebSocket message streaming (currently sends full buffer each frame)
+- Connection pooling and backpressure handling
+- Binary protocol for efficiency (instead of JSON)
+- Client management with proper async handling
+- Mark for deprecation if replaced by a new streaming implementation
+
+Current implementation: Simple broadcast of text frames to all connected clients.
 """
 
 import asyncio