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/multi.py

@@ -4,7 +4,10 @@ Multi display backend - forwards to multiple displays.
 
 
 class MultiDisplay:
-    """Display that forwards to multiple displays."""
+    """Display that forwards to multiple displays.
+
+    Supports reuse - passes reuse flag to all child displays.
+    """
 
     width: int = 80
     height: int = 24
@@ -14,11 +17,18 @@ class MultiDisplay:
         self.width = 80
         self.height = 24
 
-    def init(self, width: int, height: int) -> None:
+    def init(self, width: int, height: int, reuse: bool = False) -> None:
+        """Initialize all child displays with dimensions.
+
+        Args:
+            width: Terminal width in characters
+            height: Terminal height in rows
+            reuse: If True, use reuse mode for child displays
+        """
         self.width = width
         self.height = height
         for d in self.displays:
-            d.init(width, height)
+            d.init(width, height, reuse=reuse)
 
     def show(self, buffer: list[str]) -> None:
         for d in self.displays: