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

tests/test_websocket_e2e.py

@@ -0,0 +1,78 @@
+"""
+End-to-end tests for WebSocket display using Playwright.
+"""
+
+import time
+
+import pytest
+
+
+class TestWebSocketE2E:
+    """End-to-end tests for WebSocket display with browser."""
+
+    @pytest.mark.e2e
+    def test_websocket_server_starts(self):
+        """Test that WebSocket server starts and serves HTTP."""
+        import threading
+
+        from engine.display.backends.websocket import WebSocketDisplay
+
+        display = WebSocketDisplay(host="127.0.0.1", port=18765)
+
+        server_thread = threading.Thread(target=display.start_http_server)
+        server_thread.daemon = True
+        server_thread.start()
+
+        time.sleep(1)
+
+        try:
+            import urllib.request
+
+            response = urllib.request.urlopen("http://127.0.0.1:18765", timeout=5)
+            assert response.status == 200
+            content = response.read().decode("utf-8")
+            assert len(content) > 0
+        finally:
+            display.cleanup()
+            time.sleep(0.5)
+
+    @pytest.mark.e2e
+    @pytest.mark.skipif(
+        not pytest.importorskip("playwright", reason="playwright not installed"),
+        reason="playwright not installed",
+    )
+    def test_websocket_browser_connection(self):
+        """Test WebSocket connection with actual browser."""
+        import threading
+
+        from playwright.sync_api import sync_playwright
+
+        from engine.display.backends.websocket import WebSocketDisplay
+
+        display = WebSocketDisplay(host="127.0.0.1", port=18767)
+
+        server_thread = threading.Thread(target=display.start_server)
+        server_thread.daemon = True
+        server_thread.start()
+
+        http_thread = threading.Thread(target=display.start_http_server)
+        http_thread.daemon = True
+        http_thread.start()
+
+        time.sleep(1)
+
+        try:
+            with sync_playwright() as p:
+                browser = p.chromium.launch(headless=True)
+                page = browser.new_page()
+
+                page.goto("http://127.0.0.1:18767")
+                time.sleep(0.5)
+
+                title = page.title()
+                assert len(title) >= 0
+
+                browser.close()
+        finally:
+            display.cleanup()
+            time.sleep(0.5)