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

example/session.py

@@ -1,4 +1,4 @@
-from kaya.core import HttpContext, KayaApp
+from kaya.core import HttpContext, KayaApp, WebSocket
 from kaya.session import InMemorySessionStore, SessionMixin
 
 app = KayaApp(mixins=[SessionMixin(InMemorySessionStore())])
@@ -21,3 +21,11 @@ async def read(ctx: HttpContext) -> None:
 async def clear(ctx: HttpContext) -> None:
     ctx.session.invalidate()
     await ctx.send_str(200, 'session cleared')
+
+
+@app.websocket('/ws/visits')
+async def ws_visits(ws: WebSocket) -> None:
+    visits = ws.session.get('visits', 0) + 1
+    ws.session['visits'] = visits
+    await ws.accept()
+    await ws.send_text(f'visits: {visits}')