fix(CI-CD): don't treat a shard's zero-mutant files as a failure

[baxyz]

Summary

• The first real sharded run (2026-09-08T10:49Z, after feat(CI-CD): shard mutation-dashboard.yml so its runtime stops scaling with the helper count #162) failed at the merge step: 20 files across all 4 shards were reported "missing," when in fact Stryker simply omits any file with zero mutable code (pure type/interface declarations) from its files map entirely.
• Verified against main's own pre-sharding baseline: it already excludes the exact same 20 files (helpers/type/*.ts, and a handful of types.ts/_types.ts files) — 296 of 316 total mutable files, not a sharding-specific issue.
• merge-shards.ts no longer treats an absent file as an error; it's counted and logged as a zero-mutant skip instead. The shard-artifact-count check (catching a shard that fails before it can even upload) is untouched — that's the actual failure signal, since Stryker only ever writes its report in one complete shot at the end of a run.
• Re-verified locally with the same fixture-based tests as feat(CI-CD): shard mutation-dashboard.yml so its runtime stops scaling with the helper count #162, plus a new case mixing a zero-mutant file into a shard's manifest.

Test plan

• pnpm run typecheck / pnpm run lint clean
• Local fixture tests: zero-mutant file skipped without error; missing-shard-artifact still hard-fails
• Next push-to-main run completes end-to-end (all shards + aggregate succeed, baseline commits)

🤖 Generated with Claude Code

[github-actions]

✅ PR Validation Passed

All checks passed!

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📋 Pipeline Status

	Job	Status
✅	🔢 Version	passing
✅	🏗️ Build	passing
✅	🧪 Tests	passing
✅	📝 Lint	passing
✅	📘 TypeCheck	passing
✅	🔐 Security Audit	passing
✅	🧾 Conventional Commits	passing
✅	🔗 Coherency	passing

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📊 Code Coverage

✅ Overall Coverage: 100.0% — Target reached! 🎯

	Metric	Progress	Coverage
✅	Lines	▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓	100%
✅	Branches	▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓	100%
✅	Functions	▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓	100%
✅	Statements	▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓	100%

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🧬 Mutation Testing

⏭️ Mutation Testing: skipped

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🌐 Runtime Compatibility

✅ Runtime Compatibility: 3/3 — All runtimes passed!

	Runtime	Status
🟩	Node.js 26 (LTS) v26.8.1	✅
🦕	Deno 2.8.3	✅
🍞	Bun 1.4.2	✅

🔄 Runtime compatibility is informational only and does not block the PR

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⏱️ Benchmarks

⏭️ Benchmarks: skipped

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ℹ️ About this report

• 🎯 Coverage Target: 100% for all metrics
• 🧬 Mutation Testing: powered by Stryker (non-blocking)
• ⏱️ Benchmarks: powered by Vitest Bench (non-blocking)
• 🔄 This comment updates automatically with each push
• 📈 Coverage is measured using Vitest + v8

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_{🤖 Generated by @helpers4 CI • 2026-09-08}