ws_transport implementation over an HTTP/2 stream.

After an extended-CONNECT WebSocket handshake (RFC 8441), the WS session exchanges frames as HTTP/2 DATA on the same stream. This module maps the ws_transport callbacks onto the h2 library:

• send/2 -> h2:send_data/4 (no end_stream; frames keep flowing)
• controlling_process/2 -> h2:set_stream_handler/4, so the session process receives the stream's DATA events
• classify/2 turns {h2, Conn, {data, ...}} into {ws_data, ...} and stream resets into {ws_closed, ...}

The handle is {Conn, StreamId}.