feat(#1187): per-module Stryker mutation-score ratchet (ADR-456 80% floor) (#1200)

* feat(#1187): per-module mutation-score ratchet + graduate core-utils

ADR-456's 80% mutation floor was unenforceable as a single global break=50:
4 of 6 covered modules sit at 63-79% and forcing them to 80 would require
brittle exact-string assertions on equivalent string-literal mutants (a
Goodhart's-Law trap). Instead, each covered module declares a minScore floor
(locked at its measured score, TARGET 80) enforced per CI shard via
stryker --break, ratcheting up over time without brittle tests.
- mutation-matrix.cjs: minScore per module + TARGET_MUTATION_SCORE=80, emitted
  in the matrix; require.main guard + exports for testability.
- mutation.yml: per-shard --break <minScore>.
- stryker.config.mjs: global break 50->60 as a local backstop (CI uses minScore).
- Graduated core-utils (measured 77.5%, floor 75).
- context-utilization 79.5->92.3% via behavioral killers (state classification
  outputs + error-value contract, not exact-string matches) -> minScore 80 (TARGET).
- ratchet-integrity guard test (28 cases).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

* fix(#1187): pass mutation break via MUTATION_BREAK env (no stryker --break flag)

Adversarial review caught that Stryker 9.x has no --break CLI flag, so the
per-shard 'stryker run --break <minScore>' errored out every mutation shard.
Read the per-module floor from process.env.MUTATION_BREAK in stryker.config.mjs
and set it per shard via env in mutation.yml. Red-green verified: MUTATION_BREAK=99
exits 1, =80 exits 0. Also make the ratchet guard monotonic (RATCHET_BASELINE
floors; lowering a floor now fails the guard unless the baseline is edited).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

* fix(#1187): fail closed on bad MUTATION_BREAK + monotonic ratchet baseline

Code review: Number(env)||60 failed OPEN — an empty/invalid MUTATION_BREAK
(e.g. a future module missing minScore -> matrix expands to '') silently
degraded the shard to break 60, letting a high-floor module regress undetected.
resolveMutationBreak() now returns 60 only when the env is truly unset (local
backstop) and THROWS on present-but-empty/non-numeric/out-of-range (fail closed);
stryker.config.mjs imports it via createRequire. Also make RATCHET_BASELINE an
equality mirror (=== not >=) so any floor change is explicit in review and no
floor can be silently lowered. Tests: 46 (incl resolveMutationBreak cases).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

* fix(#1187): recalibrate config-schema/prompt-budget floors to CI scores

First CI mutation run failed two shards: the floors were set from local Stryker
runs whose TIMEOUTS were counted as kills (env-variable), inflating scores. CI
runs with timeout~0, so the real deterministic scores are lower:
- config-schema: local 69.7% -> CI 54.55% (5 local timeouts vanished) -> floor 52
- prompt-budget: local 99.6% -> CI 68.33% (239 local timeouts vanished) -> floor 66
Calibrate floors from CI (the documented source of truth) and record the lesson
in the comment so future floors aren't set from timeout-inflated local runs.
Baseline updated to match. The other 5 shards passed (deterministic CI scores
above their floors).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

stryker.config.mjs

@@ -21,6 +21,12 @@
  * to stay bounded. Full runs are for local exploration only.
  */
 
+import { createRequire } from 'node:module';
+const _require = createRequire(import.meta.url);
+// resolveMutationBreak: fail-closed resolver for MUTATION_BREAK env var.
+// undefined → 60 (local backstop); set-but-empty or non-numeric → throws.
+const { resolveMutationBreak } = _require('./scripts/mutation-matrix.cjs');
+
 // ADR-457: bin/lib/*.cjs are gitignored build artifacts (compiled from
 // src/*.cts by `npm run build:lib`, which the mutation CI job runs via `npm ci`
 // → prepare before Stryker). Stryker mutates the *built* .cjs directly — the
@@ -55,7 +61,8 @@ const UNMUTATED = [
 
 // Full test command used by local runs and as the fallback when CI does not
 // inject a per-shard command via MUTATION_TEST_CMD.
-const DEFAULT_TEST_CMD = 'node --test tests/context-utilization.property.test.cjs tests/prompt-budget.property.test.cjs tests/frontmatter.property.test.cjs tests/adr-parser.property.test.cjs tests/config-schema.property.test.cjs tests/adr-parser.test.cjs tests/active-workstream-store.test.cjs tests/active-workstream-store.unit.test.cjs tests/prompt-budget.unit.test.cjs tests/adr-parser.unit.test.cjs tests/frontmatter.unit.test.cjs';
+// Keep this list in sync with the tests arrays in scripts/mutation-matrix.cjs COVERED.
+const DEFAULT_TEST_CMD = 'node --test tests/context-utilization.property.test.cjs tests/prompt-budget.property.test.cjs tests/frontmatter.property.test.cjs tests/adr-parser.property.test.cjs tests/config-schema.property.test.cjs tests/adr-parser.test.cjs tests/active-workstream-store.test.cjs tests/active-workstream-store.unit.test.cjs tests/prompt-budget.unit.test.cjs tests/adr-parser.unit.test.cjs tests/frontmatter.unit.test.cjs tests/core-utils.test.cjs';
 
 /** @type {import('@stryker-mutator/core').PartialStrykerOptions} */
 export default {
@@ -83,10 +90,16 @@ export default {
   coverageAnalysis: 'off',
 
   // ── Thresholds ───────────────────────────────────────────────────────────────
+  // ADR-456 / issue #1187: CI passes the per-module minScore (from
+  // scripts/mutation-matrix.cjs) via the MUTATION_BREAK environment variable.
+  // Each CI shard sets MUTATION_BREAK to its module's floor so Stryker enforces
+  // the ratchet. Local runs without MUTATION_BREAK fall back to 60 (backstop).
+  // Do NOT raise the fallback here; raise individual minScore values in
+  // mutation-matrix.cjs instead.
   thresholds: {
     high: 80,
     low: 60,
-    break: 50,
+    break: resolveMutationBreak(process.env.MUTATION_BREAK),
   },
 
   // ── Incremental mode ─────────────────────────────────────────────────────────