- Add BphxGeometry with builder pattern for plate heat exchanger specs - Implement 6 heat transfer correlations: Longo2004, Shah1979/2021, Kandlikar1990, GungorWinterton1986, Gnielinski1976 - Add CorrelationSelector with validity range checking - Add compute_pressure_drop() using Calib.f_dp - Implement HeatTransferCorrelation trait for BphxCorrelation - 45 BPHX tests + 17 correlation tests passing