Rewrite HIV example with multilayer network, care cascade, and PrEP

Replaces the original Granich-style single-layer SI-with-ART model.
Main + casual ERGM layers with concurrency; four-state care cascade
calibrated to UNAIDS 95-95-95 with explicit ART re-engagement; risk-
targeted PrEP eligible on total degree or HIV+ partner; recalibrated
transmission to a realistic endemic baseline (~14% prevalence).

Author: smjenness