- 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.
The code after the 'prior session' return block was incorrectly indented
at 1 space, placing it inside the 'if is_viewing_prior_session' block
instead of after it. This caused 'total_cost' and 'perc' to be undefined
when viewing an active session, triggering an IM_ASSERT error.
Fix: Moved 'track_name', 'track_stats', and 'total_cost' to the
correct 2-space indentation (method body level).