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/terminal.py

@@ -6,17 +6,32 @@ import time
 
 
 class TerminalDisplay:
-    """ANSI terminal display backend."""
+    """ANSI terminal display backend.
+
+    Renders buffer to stdout using ANSI escape codes.
+    Supports reuse - when reuse=True, skips re-initializing terminal state.
+    """
 
     width: int = 80
     height: int = 24
+    _initialized: bool = False
 
-    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: If True, skip terminal re-initialization
+        """
         from engine.terminal import CURSOR_OFF
 
         self.width = width
         self.height = height
-        print(CURSOR_OFF, end="", flush=True)
+
+        if not reuse or not self._initialized:
+            print(CURSOR_OFF, end="", flush=True)
+            self._initialized = True
 
     def show(self, buffer: list[str]) -> None:
         import sys