- self.engine was a single ConductorEngine reference that got overwritten
when multiple tracks ran concurrently, orphaning the first track's engine
- Now uses self.engines: Dict[str, ConductorEngine] keyed by track.id
- Updated _spawn_worker, kill_worker, pause_mma, resume_mma, approve_ticket,
_load_active_tickets, and _update_ticket_depends_on to use engines.get(track_id)
Fixes concurrent MMA track execution bug where only one worker ever appeared.
