Why: waiting branches should keep their work while one candidate syncs and merges, and a conflict must not block unrelated healthy work.
Evidence: SELF three Git scenarios and governance suite captured by make evidence; real CI and three-business-task acceptance remain NOT_RUN.
Impact: local regression scenarios only; no automatic queue mutex, production merge or measured CI savings are claimed.
Refs: U6; tests/test_merge_queue_trial.py; .task/feature-merge-queue-policy-draft.md