Add websocket session support to kaya-session

- kaya-core: WebSocket ABC gains session attribute and accept(headers=...)
- kaya-core: AsgiWebSocket injects headers into websocket.accept message
- kaya-rsgi: RsgiWebSocket accepts headers param (ignored — Granian's
  accept() takes no args)
- kaya-session: SessionWebSocket wrapper exposes ws.session and injects
  Set-Cookie on accept()
- kaya-session: SessionMixin registers before/after websocket hooks;
  session loaded at connect, persisted on close if modified
- 10 new WV session tests covering read, persist, handshake cookie,
  regenerate, invalidate, isolation
- Example and README updated

packages/kaya-rsgi/src/kaya/rsgi/_rsgi.py

@@ -141,7 +141,9 @@ class RsgiWebSocket(WebSocket):
                        .map(lambda it: (it[0], int(it[1])))
                        .or_else_throw(RuntimeError))
 
-    async def accept(self) -> None:
+    async def accept(self, headers: Optional[Mapping[str, StrOrStrings]] = None) -> None:
+        # RSGI's websocket accept() takes no arguments (https://github.com/emmett-framework/granian/blob/master/docs/spec/RSGI.md),
+        # so handshake headers (e.g. Set-Cookie) cannot be sent on RSGI; they are silently ignored here.
         self._transport = await self._protocol.accept()
 
     async def receive(self) -> WebSocketMessage: