Add pytest-cov coverage reporting (informational, not a CI gate)

[Circout-sudo]

Closes #249

• Add a test extra to pyproject.toml (pytest, pytest-cov)
• Add a make coverage target; make test output is unchanged
• CI's profiler job installs the new test extra and runs a new, separate "Coverage report (informational)" step via make coverage, right after the existing make test step (which is untouched)

No --cov-fail-under is set anywhere, so this can never fail a PR on a coverage threshold - it's visibility only, as requested in the issue.

Baseline on this run: 82% overall coverage across faircode/. Weakest spots: figures.py (0%), cli.py (51%), report.py (64%), benchmark.py (72%) - good candidates for a follow-up issue.

README coverage badge (mentioned as optional in #249) is not included here since it needs a live coverage service (Codecov/Coveralls) set up by the maintainer first.

⏸️ Paper freeze active. Do not modify anything under paper/results-frozen/, results/, the faircode/ analysis core, any audit.yaml / dataset CSV, or the reproducibility parameters (random_state, split, iteration counts, metrics) - these are frozen for a paper in peer review (see CLAUDE.md). New audits cannot merge to main yet (timing hold - they will be parked post-paper). Explainers, docs, website, and tooling are unaffected. If you think you found a bug in the analysis code, flag it in an issue - do not silently fix it.

Summary

Type

• Audit
• Explainer
• Bug fix
• Other

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Audit checklist

• I opened or linked a corresponding issue first
• The folder is named after the domain, not the dataset
• unfair.py includes protected attributes and prints the required output format
• fair.py removes protected attributes and identified proxy variables
• Both scripts use random_state=42 and an 80/20 train/test split
• Proxy variables were actually tested, not just guessed
• unfair.png and fair.png are included as PNG screenshots
• The dataset is public and accessible without login or payment
• The dataset file is included, or DATA.md is included if the file is too large
• README.md includes the new results row and audit section
• A notebook was added if the audit benefits from one

Before fairness gap:

After fairness gap:

Reduction:

Protected attribute(s):

Proxy variables dropped:

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Explainer checklist

• The file is in explainers/ and uses lowercase hyphenated naming
• It includes a plain-language definition
• It uses a real example from this repo or a documented real-world case
• It includes runnable Python detection or measurement code
• It acknowledges limitations or trade-offs
• It links to related explainers or repo projects
• It includes 2-3 primary sources
• The Explainers table in README.md was updated

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Linked issue

Closes #

[vercel]

@Circout-sudo is attempting to deploy a commit to the yashkewlani2020-gmailcom's projects Team on Vercel.

A member of the Team first needs to authorize it.

[github-actions]

@yakew7 @ahmdkaml - new PR to review.