Restore code coverage threshold to 80%

## Summary

The CI coverage gate is currently `--cov-fail-under=65` (Makefile line 83). The original 80% target was relaxed several times during the airframe + Burr migration to keep CI green while the substrate shifted underneath. Now that the substrate is settled (Burr workflows + airframe Pattern D, all 9 BURR migration gaps closed), it's time to raise the floor back toward 80%.

## Tasks

- [ ] Establish the current coverage number for `main` (run `make test-cov` and record the report).
- [ ] Identify the modules pulling the floor down (likely the new Burr action layers and squadron lifecycle paths).
- [ ] Add targeted tests for genuine gaps; mark unreachable defensive branches with `pragma: no cover` where appropriate.
- [ ] Raise `--cov-fail-under` in the Makefile in steps: 65 → 70 → 75 → 80 across separate PRs so each step is reviewable.

## Notes

- The original issue cited `review_registry.py`; that module was removed during the migration, so the specific coverage debt it tracked is no longer relevant.
- Don't bypass the gate by lowering it further when adding new code — every new module should aim for ≥80% on its own.

## Acceptance Criteria

- Coverage is at or above 80% on `main`.
- `--cov-fail-under=80` is restored in the Makefile.
- CI passes with the restored threshold.

## Origin

Originally filed 2026-01-06; updated 2026-05-26 to reflect current state.

Start by running `make test-cov` and recording the current report, then inspect the Burr action layers and squadron lifecycle paths identified as likely coverage gaps. Add targeted tests or mark genuinely unreachable branches as excluded, update the threshold at Makefile line 83, and verify that coverage reaches 80% and CI passes.

Written by the indexing model from the issue text.