This module builds the MPICH 4.2.0 C runtime from its release archive.
The public @mpich//:mpich target supplies C headers and the shared libmpich.so.12 library. @mpich//:runtime exposes the installed runtime tree for consumers that load MPICH dynamically rather than linking it.
The build intentionally selects the portable CH3 socket device and omits Fortran, C++, and process-manager binaries. This keeps the module's runtime dependency surface small while preserving the MPI C ABI used by prebuilt consumers such as mpi4py.