fix(repo): pass a mutate set that matches no file with an empty report

[systemfsoftware-maker]

Summary

A stryker run whose mutate patterns match no file now performs the dry run, writes a mutation report with files: {}, and exits 0 whatever thresholds.break says. Before, it stopped at the instrument stage with exit 3 and printed InstrumentError: with an empty message, which failed sharded CI jobs whose shard owned no file.

Fixes #114

What changed for users

• Empty mutate set: dry-runs and reports like any other run, matching upstream StrykerJS. An empty run has no mutation score, so it no longer trips the break threshold.
• Printed errors: prepare, dry-run, and mutation-test stage errors carry their reason as the message, so a printed cause no longer shows up as a bare error name.
• stryker merge-reports: a package whose report has no tested mutant shows n/a instead of 0.00.
• Breaking, @systemfsoftware/stryker-js-plugin-interface (major): Report.Metrics#mutationScore and #mutationScoreBasedOnCoveredCode return Report.MutationScore (Scored { percentage } or Unscored) instead of a number that was NaN for an empty run. The changeset shows the Report.MutationScore.match migration. The machine-mode verdict envelope keeps score: number | null, and mutation.json never carried metrics, so neither wire format changes.

Why it takes this shape

The exit-code failure was not in the instrument stage alone. Once an empty set was admitted, Metrics.mutationScore returned NaN, and ClassifyExitCommand's schema rejected it (exit 1). Returning 0 would trip every configured break threshold, so the score became a union that cannot hold an invalid number. Every consumer (exit classifier, clear-text renderer, merge-reports, verdict envelope) now matches on the tag, and classifyExit is the single place that decides the exit code.

Verification

• CLI on a fixture with mutate: ["src/a.ts", "!src/a.ts"] and thresholds.break: 100: exit 0, reports/mutation/mutation.json has "files": {}. Before the fix: exit 3.
• New integration scenarios in packages/stryker-js/tests/empty-file-selection.integration.test.ts: an empty shard passes with an empty report, and an empty project fails at prepare with No input files found. as the stated cause.
• Suites: stryker-js 634/634, plugin-interface 156/156, html-reporter 4/4. Workspace pnpm typecheck 58/58. oxlint, tsgo, dprint, and api:check are clean.
• Property laws were audited for vacuity. Each new law was run against a deliberately broken implementation (< changed to <=, fileCount forced to at least 1, an unscored run failing a break threshold, the merge scoring an empty module), and all four broken versions failed.

Review findings (all applied)

Finding	Resolution
P1: merge-reports computed its score apart from Report.Metrics (merge-report-parts.workflow.ts:215)	The make-body-purity gate forbids calling Report.Metrics from the workflow's decide body, so the dead clamp that made the two differ was removed (2d132aa), and ∀cs_Modules_≡RowScoreIsTheReportMetricsScore now requires every merged row's score to equal the Report.Metrics score (191f494). A deliberately drifted merge formula fails that law.
P1: consumers outside this repo that read mutationScore as a number break (Metrics.schema.ts:53)	Intended breaking change, declared major; the changeset now shows the before/after migration (670998e).
P2: the score was rendered in two places (mutation-reporting.service.ts, clear-text renderer)	Only a scored run can break a threshold, so the exit-code logs take a plain percentage and the service's own n/a renderer is gone (86d68c3).
P3: an unscored run with no break threshold logged "No breaking threshold configured"	Unscored runs with no threshold log nothing (f7e37a0).

Review run 20260925-153149-c8aaef73: correctness, project-standards, testing, maintainability, api-contract, and an in-process adversarial lens; no cross-model peer was installed. The validator rejected three further findings with code evidence (the reviewers expected mutation.json or other packages to read the score; they do not).

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