Root cause: Hangup had no call_id field. The relay forwarded hangups to
ALL active calls for a user. When user A hung up call 1 and user B
immediately placed call 2, the relay's processing of A's hangup would
also kill call 2 (race window ~1-2s).
Fix: add optional call_id to Hangup (backwards-compatible via serde
skip_serializing_if). When present, the relay only ends the named call.
Old clients send call_id=None and get the legacy broadcast behavior.
Also: clear pending_path_report in Hangup recv handler and
internal_deregister to prevent stale oneshot channels from blocking
subsequent call setups.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
a798634b3d