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
woggioni
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