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Summary

Closes #167. The mutation report had two problems: it printed every mutant, and Stryker never really ran.

Why Stryker read as "did not finish". On identity it ran 60 of 427 tests, and almost every mutant "survived". There were two causes:

• The sandbox loses imports from outside the web directory. identity's src/app/contract.ts imports ../../../openapi/v1/openapi.yaml?raw. Stryker copies only web/, so every spec that loads the app failed to import, and the Vitest runner dropped those files without a word.
• Under Vitest 5 its Vitest runner switches no mutant on. In place, the dry run runs 379 tests and inject() delivers the active mutant to the setup file. Mutants still never activate: 237 of 241 "survive" on format.ts, a file its tests cover fully.

The fix.

• Changed lines only. Stryker mutates only the lines the pull request adds or changes. A new scripts/changed_ranges.py turns git diff -U0 into file:start-end ranges, as gremlins --diff already does for Go.
• One run per changed file, in place. Each run uses the command runner with pnpm exec vitest related --run <file>. The command runner switches a mutant on through the environment, and in place the outside imports resolve.
• Proven locally. On identity's format.ts, the command runner killed 225 of 241 mutants over the whole file. Limited to the 22 mutants on chore(main): release 0.15.1 #96's changed lines, all were caught, in 1.5 minutes.
• JSON only. The clear-text reporter is gone, so the job log no longer prints every mutant.

A shorter comment.

• Survivors. Still listed, up to 30, then "…and N more".
• Unreached mutants. Counted per file, up to 15 files, instead of one row each. chore(main): release 0.15.1 #96's comment was 30 KB, mostly regenerated client code.
• Generated code. Left out everywhere: a file whose first lines say Code generated, auto-generated or DO NOT EDIT, read against --go-root and --web-root.
• Several runs. --stryker takes one report per run.
• Coverage-off runs. stryker_worked trusts a run that killed or timed out mutants even without coverage, as the command runner reports. A run that switched nothing on still reads as not finished.

A caller no longer needs @stryker-mutator/vitest-runner, only @stryker-mutator/core and Vitest.

Testing

• changed_ranges.py and mutation_report.py are both at 100% line and branch coverage, with a new CI step for the former.
• The new tests cover:
  • ranges per hunk, deletions, deleted files;
  • per-file counts and their order and cap, the survivor cap;
  • generated files left out, and an unreadable file kept;
  • several reports merged;
  • trusting a coverage-off run, and not trusting a run that switched nothing on;
  • the workflow's command runner, in-place mode, changed-line ranges and no clear-text reporter.
• The whole suite (226 tests) and actionlint pass.
• The workflow's Stryker loop ran end to end locally on identity: ranges, jq-written config, Stryker, report.