[Phase 16] S14 — Test-quality ratchet #900

Roadmap · Phase 16, step S14 · size M · 2 unit(s) · planned in docs/roadmap/phase-16-trust-surface-hardening.md:723-744

Summary

The test suite's own quality contract is weaker than its actual quality: a 40 % coverage floor guards a measured 86.60 %, and a curated subset of tests asserts only "did not raise" and would pass against a pass body. Deliberately late in the phase, so the ratchet is not calibrated against pre-remediation code.

Units

Decisions this step executes

• C-19 — coverage ratchet: diff-coverage enforcing at S14 only (the "advisory at S1" limb was withdrawn: a CI step that cannot go red is a fake green); global floor = measured − 2.

Plan

• Introduce diff-coverage once, enforcing; ratchet the global fail_under; add per-module floors.

Exit criteria

• Diff-coverage on changed lines and a ratcheted global fail_under are both enforced, with explicit measured thresholds for compliance.py, verify/, _http.py, safety/ and config.py.
• Replacing the body of each curated production function with a no-op makes its test fail.
• The 71-function inventory is triaged by hand into legitimate no-exception contracts versus silent-pass risks, with the disposition recorded.
• The floor stated in testing.md matches the enforced one.

How to deliver

• One root-cause family, committed on its own; then an Opus review round and a Sonnet review round, each with verified findings fixed and committed before the next step begins.
• A reproducer that is red before the fix and green after; the full gauntlet from CLAUDE.md passes under ./.venv/bin/python.
• Every English documentation edit ships with its Turkish mirror in the same commit; a new or renamed audit event gets its EN and TR catalog rows in the same PR.
• A step that grows a deferred module carries its budget_history justification in the same diff; a widened guard rolls out non-strict first, then flips.
• Commit bodies cite Absorbs F-W20260729-…; the CHANGELOG [Unreleased] section gets an entry only for a change a user or library consumer can observe (decision C-20).

_{Planned in docs/roadmap/phase-16-trust-surface-hardening.md (read at f94595f). Unit IDs (F-W20260729-…) come from the 2026-07-29/30 full-project review; the issue numbers next to them are the October 2026 review's records of the same defects.}