test: ratchet mutation score over enumerated decision kernels (#1441)

* test: ratchet mutation score over enumerated decision kernels

Adds a Stryker (vitest runner) mutation lane scoped to the decision kernels,
a per-module baseline with tool/config provenance, and a ratchet that only
lets scores rise. Non-gating until two consecutive stable weekly sweeps.

Refs #1415

Co-Authored-By: Devin AI <158243242+devin-ai-integration[bot]@users.noreply.github.com>

* chore: declare the mutation test-scope seam for production-export analysis

Co-Authored-By: Devin AI <158243242+devin-ai-integration[bot]@users.noreply.github.com>

* test: own kernel tests in the mutation registry and ship the #1430 lane envelope

- restore bench:help-conformance, broken by a formatting-path edit
- kernel test files select their module on PRs (registry `tests` + workflow paths),
  asserted to reach the kernel through the import graph
- every mutation run writes the standard scheduled-lane artifact envelope
- move src/utils/__tests__/errors.test.ts beside its source per the mirror rule

Refs #1415, #1430

Co-Authored-By: Devin AI <158243242+devin-ai-integration[bot]@users.noreply.github.com>

* test: derive kernel test ownership and land the scheduled-lane health monitor

Ownership of a kernel's tests is now computed from the static import graph
(scripts/mutation/ownership.ts) instead of a hand-listed set, so a test that
reaches a kernel indirectly -- src/__tests__/daemon-error.test.ts through
src/daemon.ts -- selects that kernel on a PR. The PR lane triggers on every src
test and shards the derived modules, keeping wall clock at one module.

The lane envelope (#1430) is now written on every exit path with the stage it
reached, so a crash before any mutant runs is distinguishable from a lane that
never ran. Adds the derived cadence monitor (scripts/lane-health, daily
workflow): scheduled lanes are enumerated from .github/workflows/ and reported
dark, failing, or never-run against their own cron cadence.

* fix: merge only Stryker reports from a shard directory

The shard artifacts now carry the lane envelope beside mutation.json, and the
merge globbed every .json under the download path, so the ratchet job fed the
envelope to the report parser and died after the mutants had already run.

* fix(mutation): fail on incomplete shard sets and envelope pre-run failures

Co-Authored-By: Devin AI <158243242+devin-ai-integration[bot]@users.noreply.github.com>

* fix(mutation): downgrade a passing envelope when a later lane step fails

Co-Authored-By: Devin AI <158243242+devin-ai-integration[bot]@users.noreply.github.com>

* refactor(mutation): shard by registry, defer the PR lane, drop the bundled watcher

Co-Authored-By: Devin AI <158243242+devin-ai-integration[bot]@users.noreply.github.com>

* docs: describe registry sharding and the deferred PR mutation lane

Co-Authored-By: Devin AI <158243242+devin-ai-integration[bot]@users.noreply.github.com>

* fix(mutation): make the pre-graduation tooling exception select real mutants

Co-Authored-By: Devin AI <158243242+devin-ai-integration[bot]@users.noreply.github.com>

* test(mutation): give the worktree fixture commits their own identity

Co-Authored-By: Devin AI <158243242+devin-ai-integration[bot]@users.noreply.github.com>

---------

Co-authored-by: Michał Pierzchała <thymikee@gmail.com>
Co-authored-by: Devin AI <158243242+devin-ai-integration[bot]@users.noreply.github.com>

stryker.config.json

@@ -0,0 +1,41 @@
+{
+  "$schema": "./node_modules/@stryker-mutator/core/schema/stryker-schema.json",
+  "_comment": "Decision-kernel mutation lane (issue #1415). The mutate list below MUST mirror KERNEL_MODULES in scripts/mutation/modules.ts — scripts/mutation/config.test.ts asserts it. Ratchet verdicts come from scripts/mutation/run.ts, never from Stryker's own thresholds. Any edit here changes the config content hash, which invalidates every recorded baseline as non-comparable until re-recorded with `pnpm mutation:baseline`.",
+  "packageManager": "pnpm",
+  "tsconfigFile_comment": "Points at a path that does not exist on purpose. Stryker's sandbox rewrites tsconfig `extends`/`references` through the classic TypeScript API, which the repo's TypeScript 7 native package no longer exposes (`ts.parseConfigFileTextToJson is not a function`). Our tsconfig has neither extends nor references, so there is nothing to rewrite — skipping the preprocessor is exact, not a workaround for a real dependency. The behaviour relied on is that the preprocessor no-ops when the file is absent: see packages/core/src/sandbox/ts-config-preprocessor.ts (https://github.com/stryker-mutator/stryker-js/blob/master/packages/core/src/sandbox/ts-config-preprocessor.ts) and https://stryker-mutator.io/docs/stryker-js/configuration/#tsconfigfile-string. Re-check it on every Stryker upgrade: if a future version errors on a missing tsconfig instead, point this at a real, extends-free tsconfig.",
+  "tsconfigFile": "tsconfig.stryker-absent.json",
+  "plugins_comment": "Named explicitly: the default `@stryker-mutator/*` glob does not resolve under pnpm's non-flat node_modules, so plugin discovery finds no test runner.",
+  "plugins": ["@stryker-mutator/vitest-runner"],
+  "testRunner": "vitest",
+  "vitest": {
+    "configFile": "vitest.mutation.config.ts",
+    "related": true
+  },
+  "reporters": ["progress", "clear-text", "json"],
+  "jsonReporter": {
+    "fileName": ".tmp/mutation/mutation.json"
+  },
+  "htmlReporter": {
+    "fileName": ".tmp/mutation/mutation.html"
+  },
+  "coverageAnalysis": "perTest",
+  "ignoreStatic": true,
+  "tempDirName": ".tmp/stryker",
+  "cleanTempDir": true,
+  "timeoutMS": 20000,
+  "timeoutFactor": 2,
+  "thresholds": {
+    "high": 90,
+    "low": 70,
+    "break": null
+  },
+  "mutate": [
+    "src/kernel/errors.ts",
+    "src/daemon/ref-frame.ts",
+    "src/commands/interaction/runtime/settle.ts",
+    "src/utils/scroll-edge-state.ts",
+    "src/selectors/**/*.ts",
+    "!src/selectors/**/*.test.ts",
+    "!src/selectors/__tests__/**"
+  ]
+}