feat(benchmark): mutation-validity lint — flag inert (no-defect) mutations

[jobordu]

Deep-research biggest-lever item. ~100 challenges have a file-modify mutation that only bare-appends // modified by benchmark — pure file churn creating no detectable defect. 95 are correctly tagged infeasible/fictional (excluded from the capability metric), but 5 are tagged feasible — miscategorised: they claim to be detectable yet can never pass, silently dragging the capability score.

bin/mutation-validity.cjs classifies inert mutations and surfaces the miscategorised class (exit 1 if any feasible inert remain), so they get a real mutation or an honest re-tag — and future inert padding is caught.

Mutation validity — 100 inert (no-defect) mutations:
  infeasible  63  (excluded — ok)
  feasible    5   ← MISCATEGORISED (BENCH-026/029/040/044/079)
  fictional   32  (excluded — ok)

npm run mutation-validity; 3 tests. Converting the 5 (and reviewing the 95) to real anchored mutations is the tracked follow-on this lint makes visible and gate-able.

🤖 Generated with Claude Code

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📥 Commits

Reviewing files that changed from the base of the PR and between 6a37821 and 9173780.

📒 Files selected for processing (3)

• bin/mutation-validity.cjs
• package.json
• test/mutation-validity.test.cjs

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🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch feat/mutation-validity-lint

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