"""Measure phase (PRD §6.5) — the loop-closer.

    metrics.py    fleet metrics + persisted baseline snapshots (T01)
    effect.py     before/after per-pattern effectiveness (T02)
    __main__.py   python -m session_memory.measure (T03)

Computation over existing digests (reusing WP-0005 tool buckets + WP-0006 error
mining); no new capture.
"""