Render a 4-panel regional LocusCompare diagnostic for one (lead variant,
exposure study, outcome study) tuple - overlays GWAS Manhattan, QTL Manhattan,
GENCODE gene track, and cross-trait scatter colored by LD r². Use when an
agent needs visual confirmation that two GWAS / QTL signals share the same
causal variant (the Liu 2019 LocusCompare convention). Inputs: lead variant +
two pre-fetched harmonised sumstats slices (or eQTL Catalogue / GWAS Catalog
identifiers for bundled fetch). Output: PNG + JSON manifest.
