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/__init__.py

@@ -17,13 +17,30 @@ from engine.display.backends.websocket import WebSocketDisplay
 
 
 class Display(Protocol):
-    """Protocol for display backends."""
+    """Protocol for display backends.
+
+    All display backends must implement:
+    - width, height: Terminal dimensions
+    - init(width, height, reuse=False): Initialize the display
+    - show(buffer): Render buffer to display
+    - clear(): Clear the display
+    - cleanup(): Shutdown the display
+
+    The reuse flag allows attaching to an existing display instance
+    rather than creating a new window/connection.
+    """
 
     width: int
     height: int
 
-    def init(self, width: int, height: int) -> None:
-        """Initialize display with dimensions."""
+    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: If True, attach to existing display instead of creating new
+        """
         ...
 
     def show(self, buffer: list[str]) -> None: