Report coverage and artifact-mutation evidence

[Anionix]

Summary

• add a manual, pinned branch-coverage workflow and upload immutable coverage evidence
• report descriptive generated artifact-mutation completion/failure outcomes without changing rankings or gates
• exclude named boundary cases by requiring the persisted mutation recipe identity
• record the time-bounded accepted-risk exit contract in [DFL-AUDIT-121 follow-up] Set coverage gate and source-mutation baseline #506

Contract

• Lifecycle: DISCOVERED -> ENCODED -> ROUNDTRIP_VERIFIED -> BENCHMARKED -> REPORTED
• Artifact-mutation evidence is descriptive and non-rankable.
• Primary sources: coverage.py 7.10.7 branch measurement documentation and the uv run --with contract, linked from README and workflow comments.
• NO_LEAN: this slice changes Python/reporting/config only and introduces no Lean definitions or proofs.

Verification

Exact head: 7f124d963ceba7f06ff180ac8d23e6e2b1f226d0

• PYTHONPATH=src /Users/st/Documents/data-format-lab/.venv/bin/pytest tests/test_report.py tests/test_robustness_summary.py -> 12 passed
• uvx ruff check src/format_bench/report.py src/format_bench/robustness/summary.py tests/test_report.py tests/test_robustness_summary.py -> clean
• uvx ty check --python /Users/st/Documents/data-format-lab/.venv src/format_bench/report.py src/format_bench/robustness/summary.py -> clean
• workflow YAML parsed and contains the coverage job
• fixed-SHA spec review: no findings
• fixed-SHA standards/code-smell review: no findings

Post-merge: dispatch the manual coverage workflow on the merged main SHA and verify the named artifact before closing #303.

Refs #303
Refs #506

[chatgpt-codex-connector]

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