feat(websocket): native RSGI WebSocket support

[ZhuchkaTriplesix]

Context

After #88 drops the ASGI bridge, OxyRoute has no WebSocket support. RSGI exposes a first-class WebSocket protocol (RSGIWebsocketProtocol with accept / close and RSGIWebsocketTransport with receive / send_str / send_bytes); we want to dispatch WS connections through the same Rust router as HTTP.

Goal

• Register WebSocket routes via @app.websocket(path).
• When scope.proto == "websocket" the native dispatcher resolves the path against a separate router and either runs the user handler with a thin WebSocket helper or closes 1000.
• Keep the helper minimal (accept / receive / receive_text / receive_bytes / send_text / send_bytes / close); subprotocols and close codes follow Granian's RSGI semantics.

Scope

• src/state.rs: add WS route table (ws_routes, ws_router plus compiled snapshot).
• src/lib.rs: add add_websocket_route(path, handler, is_async).
• src/dispatch.rs: handle proto == "websocket" (route match → call handler with WebSocket(protocol) or close 1000).
• oxyroute/websocket.py: WebSocket helper class wrapping the RSGI protocol/transport.
• oxyroute/app.py: @app.websocket(path) decorator delegating to native registration.
• oxyroute/__init__.py: export WebSocket.
• tests/test_websocket_native.py: connect / accept / echo / close / disconnect-before-accept (Granian e2e).
• docs/websocket.md: native RSGI guide.

Acceptance criteria

• Granian e2e test passes against --interface rsgi over ws:// .
• Helper covers the common cases without leaking RSGI internals.
• README/feature.md updated to mark WebSocket as supported.

Depends on

• refactor(asgi): drop ASGI compatibility bridge #88 (drop ASGI bridge first)