Fix stale Output Interpretation numbers in underdiagnosis-bias.md

[propcgamer20-png]

Closes #523.

The Output Interpretation prose under the detection-code block quoted numbers from an earlier version of the code that were never regenerated after an edit - the same stale-paste pattern as the parallel reject-inference.md issue.

Actual output of the file's own python block

Seeded with np.random.seed(42), byte-identical across repeated runs:

figure	doc claimed	code actually prints
true_recall, Group A vs B	80.5% vs 41.2%	62.5% vs 29.2%
true false-negative gap	39.3 points	33.3 points
obs_recall, Group A vs B	~0.85 vs ~0.76	~0.64 vs ~0.33
Q4 biomarker-band diagnosis rate	0.887 / 0.536	0.785 / 0.497

Change

• Rewrote the two interpretation bullets with the real figures.
• Adjusted the item 1 framing: observed-label recall already shows a Group A/B gap in the real output, so the "the model appears to perform decently across both groups" line no longer described what the code prints. The core teaching point (standard evaluation scores the true-disease false negatives as correct True Negatives) is unchanged.
• Regenerated underdiagnosis-bias.html, faircode/_explainers/underdiagnosis-bias.md, and the llms-full.txt section via scripts/build_explainers.py.

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

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@yakew7 @Shreyash0712 - new PR touching a path you own, please review.