refactor: modularize display backends and add benchmark runner

- Create engine/display/ package with registry pattern
- Move displays to engine/display/backends/ (terminal, null, websocket, sixel)
- Add DisplayRegistry with auto-discovery
- Add benchmark.py for performance testing effects × displays matrix
- Add mise tasks: benchmark, benchmark-json, benchmark-report
- Update controller to use new display module

engine/controller.py

@@ -3,18 +3,25 @@ Stream controller - manages input sources and orchestrates the render stream.
 """
 
 from engine.config import Config, get_config
-from engine.display import MultiDisplay, NullDisplay, SixelDisplay, TerminalDisplay
+from engine.display import (
+    DisplayRegistry,
+    MultiDisplay,
+    NullDisplay,
+    SixelDisplay,
+    TerminalDisplay,
+    WebSocketDisplay,
+)
 from engine.effects.controller import handle_effects_command
 from engine.eventbus import EventBus
 from engine.events import EventType, StreamEvent
 from engine.mic import MicMonitor
 from engine.ntfy import NtfyPoller
 from engine.scroll import stream
-from engine.websocket_display import WebSocketDisplay
 
 
 def _get_display(config: Config):
     """Get the appropriate display based on config."""
+    DisplayRegistry.initialize()
     display_mode = config.display.lower()
 
     displays = []