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

pyproject.toml

@@ -75,6 +75,7 @@ addopts = [
 ]
 markers = [
     "benchmark: marks tests as performance benchmarks (may be slow)",
+    "e2e: marks tests as end-to-end tests (require network/display)",
 ]
 filterwarnings = [
     "ignore::DeprecationWarning",