feat(dgw): stream reconnecting WebM sessions

[irvingouj@Devolutions (irvingoujAtDevolution)]

Gateway WebM shadowing now consumes a recording event stream so clips can reconnect and change size.
The Start/Pull codes stay valid for existing clients.
Push disconnect marks the session ended so viewers receive StreamEnded.

Co-authored-by: Copilot 223556219+Copilot@users.noreply.github.com

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[irvingouj@Devolutions (irvingoujAtDevolution)]

Implementation notes:

• New stream_session path: recording events -> normalizer -> protocol.
• Gateway stream_recording always uses that path for WebM. webm_stream remains for tests/examples.
• Reconnect keeps the same watch stream state and appends a new clip.
• handle_disconnect sets ended = true so viewers get StreamEnded instead of waiting for TTL remove.
• Pipelined Pulls are queued. Extra Start while waiting is still rejected.
• Legacy clients still speak Start=0 / Pull=1. New server messages are Segment started (1) and Stream ended (3).

Note

LLM-assisted content (no human feedback).