StreamSourceSequenceAdvance

Server event: fires when the engine has handed off to a new active source. The pin in this message is whichever pin is now feeding the output.

last_seen_pts_us, when present, is the last PTS (microseconds) the engine emitted from the PREVIOUS active pin — the source that just ended. Absent on the very first Advance (no previous source). Clients use this for cross-switch PTS continuity: feed it back as anchor_pts_us on a sibling switch’s release_preloaded.