fix(imports): break models<->dag_engine circular dependency

Track.get_executable_tickets (in models.py) called TrackDAG at
runtime, forcing a top-level import of src.dag_engine into models.py
and creating a 2-cycle that broke whichever module loaded second
(Ticket was not yet defined when models.py loaded first; TrackDAG
was not yet defined when dag_engine.py loaded first).

Fix: hoist the method out of the Track dataclass and into a free
function get_executable_tickets(track) in dag_engine.py. models.py
no longer needs TrackDAG at all, so the cycle is one-directional
(models -> dag_engine) and resolves cleanly in any import order.

Tests updated:
- tests/test_mma_models.py: import get_executable_tickets and call
  it instead of track.get_executable_tickets() (4 call sites)
- tests/test_conductor_engine_v2.py: comment update

Verified both import orders resolve cleanly:
  forward:  import src.models; import src.dag_engine  -> OK
  reverse:  import src.dag_engine; import src.models  -> OK
34 tests pass (test_mma_models, test_dag_engine, test_execution_engine,
test_arch_boundary_phase3, test_track_state_schema).

tests/test_conductor_engine_v2.py

@@ -55,7 +55,7 @@ def test_conductor_engine_run_executes_tickets_in_order(monkeypatch: pytest.Monk
   vlogger.log_state("T1 Status Final", "todo", ticket1.status)
   vlogger.log_state("T2 Status Final", "todo", ticket2.status)
   
-  # Track.get_executable_tickets() should be called repeatedly until all are done
+  # get_executable_tickets(track) should be called repeatedly until all are done
   # T1 should run first, then T2.
   assert mock_lifecycle.call_count == 2
   assert ticket1.status == "completed"