hydro-core: shared types, the HydroModel trait, and evaluation metrics
for the multi-model hydrology platform (新安江 / TOPMODEL / SWAT).
Each model crate (hydro-xinanjiang, hydro-topmodel, hydro-swat) depends
only on this crate and implements HydroModel. The platform crate
(hydro-platform) uses DynHydroModel for heterogeneous comparison.