feat(audit): add CoV flagging with --max-cov threshold

[kaihowl]

Summary

• Adds --max-cov <percentage> CLI flag to git perf audit to flag measurements with high Coefficient of Variation (CoV = σ/μ × 100%)
• When tail or head CoV exceeds the threshold, a ⚠️ High CoV: tail=X.X%, head=Y.Y% (threshold: Z.Z%) warning is appended to the audit output
• Configurable via .gitperfconfig as max_cov under [measurement] or per-measurement tables, with standard CLI > config precedence
• Warning appears regardless of pass/fail status — it's informational about measurement stability

Closes #326

Test plan

• cargo nextest run -- --skip slow --skip test_remove — 382/382 pass
• cargo clippy — clean
• cargo fmt — applied
• Manpages regenerated via ./scripts/generate-manpages.sh
• 6 new unit tests covering: warning fires above threshold, absent below threshold, absent with no threshold, not shown for single measurement, shows both tail/head values, shown on failing audit

https://claude.ai/code/session_01RN3hLyQDXCQVLBGC3XcYCM

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Generated by Claude Code

[github-actions]

Performance Report

⏱ Performance Results

Audit Results

Error: No measurement for HEAD.
Audit failed, but continuing...

Measurement Storage Size

Live Measurement Size Report
============================

⚠️  Shallow clone detected - measurement counts may be incomplete (see FAQ)

Number of commits with measurements: 325
Total measurement data size (on-disk (compressed)): 451.5kB

Repository Statistics (for context):
-------------------------------------
  Loose objects: 0 (0B)
  Packed objects: 10809 (2.7MB)
  Total repository size: 2.7MB

Created by git-perf

[github-actions]

🧬 PR Mutation Testing Results

Status: ❌ Missed mutants detected - tests need improvement

Mutation Testing Metrics (Changed Code Only)

Metric	Count
Total Mutants	29
✅ Caught	12
❌ Missed	14
⏱️ Timeout	0
🚫 Unviable	3

Mutation Score: 46.1% (12/26 viable mutants caught)

⚠️ Action Required

This PR has 14 uncaught mutant(s) in the changed code.

What this means: The tests do not adequately verify the behavior of the code changes.

Next steps:

1. Review the mutation testing artifacts (download below)
2. Add or improve tests to catch the missed mutants
3. Run cargo mutants --in-diff <diff-file> locally to iterate faster

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Incremental mutation testing only tests code modified in this PR.
Full mutation testing runs weekly to catch issues in unchanged code.

📦 Artifacts

Download full mutation report