test(cli): a canary mutation scope that fails if Stryker stops counting a load failure as a kill

[blafourcade]

🎯 What & why

Stryker's vitest runner scores a mutant that stops a module from loading as survived, in every release up to 10.0.0. Upstream tracks it as stryker-js#6150, where the 10.0.0 repro is now posted. No Stryker option or runner option changes it. #851 fixed it here with a vitest reporter, and the suite proves what a real vitest run hands Stryker's collection. Nothing proved the last step: Stryker itself scoring that mutant Killed (#856). So a Stryker or vitest upgrade that bypassed the reporter would bring the false survivors back silently.

🛠️ How it works

• cli/tests/fixtures/stryker-canary/settings.ts holds exactly one mutant: a static StringLiteral whose replacement makes JSON.parse throw on import. Its unit test checks the unmutated value and runs in every ordinary suite.
• mutation-scopes.json declares a canary scope over it, with a floor of 100 and the reason in $comment; package.json gets test:mutation:canary.
• mutation-scopes-to-run.mjs runs it on a change to the harness (package.json, the lockfile, the Stryker config), to tests/helpers/ or to the fixture, and not otherwise.
• mutation-covers-source.arch.test.ts: "every scope matches a file" now also looks under tests/fixtures, the one place a scope may mutate something that is not product.

🧪 How to verify

• Inventory: stryker run --mutate tests/fixtures/stryker-canary/settings.ts gives one mutant, Killed, Unexpected end of JSON input.
• Reporter neutralized: Survived with testsCompleted 0. Through the gate, node scripts/run-mutation.mjs canary --force exits 1 with mutation score 0.0 is below the 100 declared in mutation-scopes.json.
• Restored: Report: reports/mutation/canary/ (score 100.0, floor 100).
• Arch red first: with the scope added and the check unchanged, scope "canary" ... matches no file. Green after.
• CHANGED=cli/package.json and CHANGED=cli/tests/helpers/unloadable-file-as-failed-test.ts select canary; CHANGED=cli/src/kernel/paths.ts gives ["kernel"].
• Architecture + fixture test 25 files, 131 tests; typecheck, lint, knip green; scripts suite green.

⚠️ Heads-up

• One more mutation job, on harness changes only. The scope's single static mutant only reruns the fixture's own test.
• When stryker-js#6150 is fixed upstream, the canary stays green without the reporter. That is the signal to delete tests/helpers/unloadable-file-as-failed-test.ts and keep the canary.

🔗 Linked issue

Closes #856

✅ I certify

• I DO CERTIFY I READ EACH LINE OF THE PULL REQUEST BECAUSE I AM A SOFTWARE ENGINEER, NOT A AI PUPPY.

🤖 Generated with Claude Code

https://claude.ai/code/session_011x4ms5qcGuZgYhCxfdHMUb