ADR-0028 (query plans / `explain`) is fully implemented; the
handoff-16 design trio (ADR-0026 / 0027 / 0028) is now closed.
- handoff-21: session summary, the two deliberate deviations
from handoff-20's plan, test coverage, open clusters.
- requirements.md: QA1 / QA2 ticked.
- CLAUDE.md: the `EXPLAIN QUERY PLAN` deferred-items line
updated to "implemented per ADR-0028".
claude@clouddev1