feat(display): add reuse flag to Display protocol

- Add reuse parameter to Display.init() for all backends
- PygameDisplay: reuse existing SDL window via class-level flag
- TerminalDisplay: skip re-init when reuse=True
- WebSocketDisplay: skip server start when reuse=True
- SixelDisplay, KittyDisplay, NullDisplay: ignore reuse (not applicable)
- MultiDisplay: pass reuse to child displays
- Update benchmark.py to reuse pygame display for effect benchmarks
- Add test_websocket_e2e.py with e2e marker
- Register e2e marker in pyproject.toml

engine/display/backends/websocket.py

@@ -86,12 +86,20 @@ class WebSocketDisplay:
         """Check if WebSocket support is available."""
         return self._available
 
-    def init(self, width: int, height: int) -> None:
-        """Initialize display with dimensions and start server."""
+    def init(self, width: int, height: int, reuse: bool = False) -> None:
+        """Initialize display with dimensions and start server.
+
+        Args:
+            width: Terminal width in characters
+            height: Terminal height in rows
+            reuse: If True, skip starting servers (assume already running)
+        """
         self.width = width
         self.height = height
-        self.start_server()
-        self.start_http_server()
+
+        if not reuse or not self._server_running:
+            self.start_server()
+            self.start_http_server()
 
     def show(self, buffer: list[str]) -> None:
         """Broadcast buffer to all connected clients."""