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fix twisted websocket close handshake (#42)

The twisted WebSocketChannel never completed the closing handshake:
a client-initiated close was not answered with a close frame, the TCP
connection was never terminated, and Request.finish() wrote its
never-started HTTP response into the upgraded websocket stream, which
clients decoded as a malformed (fragmented) close frame. Clients ended
up burning their close timeout and, depending on thread timing, failing
inside their own close logic.

Complete the handshake per RFC 6455 section 7: echo the close frame,
mark the request as written before finishing so twisted emits nothing,
and terminate the TCP connection.

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

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