docs: correct COMPAS reduction figure repo-wide (71% -> 82%) + Benefits Denial proxy count

[propcgamer20-png]

Two Results-at-a-Glance inaccuracies, one of which turned out to be repo-wide.

#545 - COMPAS "Reduction" uses the wrong formula (repo-wide)

Every audit's "Reduction" column is a relative reduction, (before - after) / before. COMPAS's alone was the raw point gap:

(86.77 - 15.69)          = 71.08  ->  "71%"    # what was printed everywhere
(86.77 - 15.69) / 86.77  = 81.9%  ->  "82%"    # the relative reduction the other 6 audits use

The other rows all verify against the relative formula (97.3%, 73.6%, ~60%, ~72%, 53%). The issue names the two summary tables, but grep shows "71% reduction" (and "71% of the gap") is COMPAS's number in 9 explainers + CHANGELOG.md too, so fixing only the tables would make the repo more inconsistent. This PR corrects every source reference and regenerates the explainer pages / faircode/_explainers/ mirror / llms-full.txt:

• COMPAS/README.md, root README.md (summary row + "Result: N% reduction" line)
• CHANGELOG.md, .github/PULL_REQUEST_TEMPLATE.md (example hint)
• explainers: ai-objectivity-myth, confounding-variable (×3), data-leakage, equalized-odds, how-ai-detects-patterns, ml-bias, proxy-variables (×2), reinforcement-learning (×3)

The underlying gap numbers (86.77%, 15.69%) are untouched - only the derived reduction % changes. This was understating COMPAS's mitigation effectiveness.

#546 - Benefits Denial README omits fnlwgt from the proxy list

audit.yaml proxy_features has five entries (marital.status, occupation, relationship, hours.per.week, fnlwgt); fair.py removes all five with its own documented reasoning. Both READMEs listed four. Added fnlwgt to Benefits Denial/README.md, the root README summary row, the "four proxy variables" -> "five" counts (2 spots), and a # fnlwgt removed line in the fair.py code-block comment.

Checks

check_em_dash.py, check_broken_links.py, check_generated_files_current.py all pass.

Closes #545
Closes #546

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