Add 4 new tests to test_dag_progress.py:
- test_production_dag_shape: Real 15-task pipeline topology with mixed
statuses, verifying all tasks present, topological order invariant,
and correct parent relationships (e.g. finalize has 4 parents)
- test_topological_sort_invariant_complex_dag: 7-node DAG with wide
branching/merging to stress-test that all parents precede children
- test_logging_throttled_by_interval: Mocks time.monotonic to verify
ctx.log() is throttled by interval while broadcasts are not
- test_uses_broadcast_event_not_append_event_and_broadcast: Verifies
progress uses transient broadcast_event, not persisted append variant
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