StrykerJS: measure test quality

Use mutation testing to ask whether tests detect plausible defects in production code. A passing suite or high coverage alone does not answer that question. Mutate production source; improve the tests that fail to detect meaningful mutations.

Choose the work

• Introduce Stryker: read setup.md, adapt the assets, and verify the integration with a small real module before widening the scope.
• Review or improve tests locally: read evaluate-tests.md. Run mutations, inspect the surviving changes and covering tests, then improve the missing inputs or assertions.
• Add CI: copy mutation-diff.yml to .github/workflows/mutation-diff.yml, along with the selector scripts and package commands. CI must pass --diff-only.

Retain the project's runner, configuration, workspace layout and package manager. The supplied integration targets pnpm, Node.js 24+ and Vitest; use the appropriate Stryker runner for Jest/Mocha rather than migrating tests just to use these snippets. Match @stryker-mutator/core and runner versions and check their actual peer dependencies.

CI and local scope