feat: add SixelDisplay backend for terminal graphics

- Implement pure Python Sixel encoder (no C dependency)
- Add SixelDisplay class to display.py with ANSI parsing
- Update controller._get_display() to handle sixel mode
- Add --display sixel CLI flag
- Add mise run-sixel task
- Update docs with display modes

engine/controller.py

@@ -3,6 +3,7 @@ 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.effects.controller import handle_effects_command
 from engine.eventbus import EventBus
 from engine.events import EventType, StreamEvent
@@ -14,12 +15,29 @@ from engine.websocket_display import WebSocketDisplay
 
 def _get_display(config: Config):
     """Get the appropriate display based on config."""
-    if config.websocket:
+    display_mode = config.display.lower()
+
+    displays = []
+
+    if display_mode in ("terminal", "both"):
+        displays.append(TerminalDisplay())
+
+    if display_mode in ("websocket", "both"):
         ws = WebSocketDisplay(host="0.0.0.0", port=config.websocket_port)
         ws.start_server()
         ws.start_http_server()
-        return ws
-    return None
+        displays.append(ws)
+
+    if display_mode == "sixel":
+        displays.append(SixelDisplay())
+
+    if not displays:
+        return NullDisplay()
+
+    if len(displays) == 1:
+        return displays[0]
+
+    return MultiDisplay(displays)
 
 
 class StreamController: