Mutation gate passes a package whose mutated set is empty (CONST-T3)

[ryanleecode]

Problem

pnpm mutation passes for a package whose mutate globs match no mutable code, for example a package that holds only schema declarations. CONST-T3 says "let an empty mutated set pass" is forbidden, but no command enforces it, so the rule is ungated.

Evidence

• @systemfsoftware/stryker-js@13.1.0 CHANGELOG.md:102: a run whose mutate patterns match no file "finishes successfully ... and passes regardless of thresholds.break".
• The runner logs "No valid mutant was tested, so there is no mutation score to hold against break threshold" and exits 0 (MutationScore.Unscored).
• @systemfsoftware/stryker-ignorer-effect-schema-declarations ignores mutants inside schema declarations, so a schema-only package reaches zero mutants even when its globs match files.

Expected

A package's mutation task fails when zero mutants were tested, unless the package opts out explicitly in its mutation config (the escape hatch CONST-T13 describes for test-set verdicts).

Acceptance

• A package whose src contains only *.schema.ts files fails pnpm mutation with a message naming the empty mutated set.
• A package with at least one killed mutant still passes.
• The opt-out is a visible config field, not a suppression comment.