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/sixel.py

@@ -271,7 +271,14 @@ class SixelDisplay:
 
         return None
 
-    def init(self, width: int, height: int) -> None:
+    def init(self, width: int, height: int, reuse: bool = False) -> None:
+        """Initialize display with dimensions.
+
+        Args:
+            width: Terminal width in characters
+            height: Terminal height in rows
+            reuse: Ignored for SixelDisplay
+        """
         self.width = width
         self.height = height
         self._initialized = True