StreamSourceSequenceSourceEnded

Server event: fires when the engine’s currently-active source fully drained (natural EOF or synthetic via end_current_source). ended_pin is the pin that just ended; last_seen_pts_us is the last PTS (microseconds) the engine emitted from it. Clients use last_seen_pts_us as the anchor_pts_us on the destination switch’s release_preloaded for PTS-continuous handoff (same-mode SSQ→SSQ or cross-mode SSQ→SSS).