feat: harden bca terminal readback correlation

[CondorCommodore]

Summary

• require a single exact terminal BCA receipt instead of scanning terminal states loosely
• return the exact correlated terminal receipt from the coord client and use it in the assignment coordinator
• add focused client/coordinator tests for duplicate terminal receipts and exact terminal receipt selection

Testing

• python3 -m pytest -q tests/test_c2_coord_client.py tests/test_cos_assignment_coordinator.py

Follow-up seam

• coord-api BCA delivery rows still do not expose cli_session_id, so this PR hardens correlation on the authoritative fields currently returned by the API and leaves full CLI-session tuple enforcement as a cross-repo follow-up