SOURCES: name the public PanCanAtlas artifacts, not the cluster path

SOURCES: cite the scientific origin, not the internal staging path

HLA: union TCGA PanImmune (primary) + OptiType fill -> 96.5% donor coverage

Add donor-level HLA class-I typing (OptiType) as metadata.hla.tsv

TCGA tumour AIRR (TCR/BCR) repertoires: 9591 samples + clinical/depth metadata