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_display.py

@@ -155,8 +155,8 @@ class TestMultiDisplay:
 
         assert multi.width == 120
         assert multi.height == 40
-        mock_display1.init.assert_called_once_with(120, 40)
-        mock_display2.init.assert_called_once_with(120, 40)
+        mock_display1.init.assert_called_once_with(120, 40, reuse=False)
+        mock_display2.init.assert_called_once_with(120, 40, reuse=False)
 
     def test_show_forwards_to_all_displays(self):
         """show forwards buffer to all displays."""
@@ -199,3 +199,12 @@ class TestMultiDisplay:
         multi.show(["test"])
         multi.clear()
         multi.cleanup()
+
+    def test_init_with_reuse(self):
+        """init passes reuse flag to child displays."""
+        mock_display = MagicMock()
+        multi = MultiDisplay([mock_display])
+
+        multi.init(80, 24, reuse=True)
+
+        mock_display.init.assert_called_once_with(80, 24, reuse=True)