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).

src/dag_engine.py

@@ -89,7 +89,7 @@ class TrackDAG:
   Returns a list of tickets that are in 'todo' status and whose dependencies are all 'completed'.
   Returns:
   A list of Ticket objects ready for execution.
-  [C: src/models.py:Track.get_executable_tickets, tests/test_dag_engine.py:test_get_ready_tasks_branching, tests/test_dag_engine.py:test_get_ready_tasks_linear, tests/test_dag_engine.py:test_get_ready_tasks_multiple_deps, tests/test_orchestration_logic.py:test_track_executable_tickets]
+  [C: src/dag_engine.py:get_executable_tickets, tests/test_dag_engine.py:test_get_ready_tasks_branching, tests/test_dag_engine.py:test_get_ready_tasks_linear, tests/test_dag_engine.py:test_get_ready_tasks_multiple_deps, tests/test_orchestration_logic.py:test_track_executable_tickets]
   """
   ready = []
   for ticket in self.tickets:
@@ -162,6 +162,17 @@ class TrackDAG:
     raise ValueError("Dependency cycle detected")
    return result
 
+def get_executable_tickets(track: "Track") -> List[Ticket]:
+ """
+ Convenience: returns the ready-to-execute tickets of a Track.
+ Free function (instead of Track.get_executable_tickets) so that
+ src/models.py does not need to import TrackDAG at module level,
+ breaking the models<->dag_engine circular dependency.
+ [C: tests/test_mma_models.py:test_track_get_executable_tickets, tests/test_mma_models.py:test_track_get_executable_tickets_complex]
+ """
+ return TrackDAG(track.tickets).get_ready_tasks()
+
+
 class ExecutionEngine:
  """
  A state machine that governs the progression of tasks within a TrackDAG.