v21 evidence-upgrade figure validation

PASS	H1 eligible target denominator
PASS	H2 unique targets
PASS	H2 five balanced folds
PASS	Safe external feature count
PASS	Primary reliability strata
PASS	scGPT denominator
PASS	GEARS denominator
PASS	PDF font embedding mode
PASS	SVG text preserved

Visual QC artifacts:
- figure3_and_s13_v21_combined_preview.png
- figure3_and_s13_v21_grayscale_qc.png

Interpretive guardrails:
- Full unique-target OOF coverage does not imply external-context validation.
- The train-mean improvement is small and has no adjusted reliability moderation.
- GEARS coverage is non-exchangeable for two audited availability variables.
- Guide-pair reliability remains exploratory (n=133).