gate(mutation): ratchet graduates on stability alone — a 28.75% score can lock in as the permanently gated floor

[thymikee]

Context

The mutation gate's graduation logic in scripts/mutation/ratchet.ts promotes a decision kernel from advisory to gating when its score is stable for requiredStableRuns: 2 consecutive weekly sweeps (mutation-baselines/decision-kernels.json, mutation-weekly.yml, mutation-affected.yml; umbrella #1412 / gate issue #1415). There is no minimum-score condition. Verified consequence (2026-07-28 audit, adversarially confirmed): scroll-edge-state sits at 28.75% (228 survived mutants) — if that number is merely stable twice, it graduates and the ratchet then enforces 28.75% as the protected floor forever. Stability of a bad score is not quality; the gate would begin blocking unrelated PRs to defend a floor that detects almost nothing.

Companion issues #1455 / (kernel-errors) raise the actual scores; this issue fixes the policy so graduation can't outrun them.

Proposed change (needs maintainer sign-off on the threshold)

Add a minimum-score condition to graduation: a kernel becomes gating only when score >= FLOOR (proposal: 70%) AND stability is met. Below-floor kernels stay advisory and are reported (in the weekly summary output) as "stable but below graduation floor".

Acceptance criteria

• ratchet.ts graduation requires both conditions; floor is a named constant in the baseline JSON schema (per-kernel override allowed, default 70) so raising/lowering it is a data change with an audit trail, not code.
• Unit tests in scripts/mutation/*.test.ts (the existing mutation:test lane) cover: below-floor+stable stays advisory; at-floor+stable graduates; already-graduated kernels are unaffected by the new rule (no regression of existing gating).
• Weekly summary output distinguishes "advisory: unstable" from "advisory: below floor" so the state is visible, not silent.
• Docs touched: docs/agents/testing.md section describing graduation criteria updated in the same PR.
• pnpm mutation:test, pnpm check:tooling green. No baseline regeneration needed (policy change only).

[thymikee]

Recommended policy (2026-07-28, revised after maintainer review — the earlier 5-part version over-built the justification mechanism; superseded):

One small PR:

1. Graduation = stability AND score ≥ 85% (floor as a constant in the baseline JSON, per-kernel override allowed). Keep requiredStableRuns: 2. Below-floor-but-stable kernels stay advisory and are labeled "below floor" in the weekly summary.
2. Survivor justification uses Stryker's native mechanism, not new machinery: a surviving mutant in a graduated kernel is either killed or annotated in-code with // Stryker disable next-line <mutator>: <reason>. Annotations move with the code (no staleness problem), appear in review diffs (no ownership problem), and the reported score then already means "unjustified survivors only". No ledger JSON, no custom validation.
3. Three sentences of docs in the same PR (docs/agents/testing.md): what qualifies a module as a decision kernel; new kernels enter at the floor; mutation gating stays kernel-scoped — never repo-wide.

Acceptance criteria in the issue description still apply except the ledger idea; tests for the ratchet change live in the existing mutation:test lane. Sequencing: #1455/#1456 land first so no current kernel is stranded below the floor.