test: add unit tests for 9 HPC modules missing coverage

[adolago]

Summary

9 HPC modules have complete implementations but zero unit tests. Adding tests will increase confidence for GA release.

Modules needing tests

• src/modules/hpc/common.rs — HPC baseline config (limits, sysctl, dirs)
• src/modules/hpc/facts.rs — HPC fact gathering (CPU, NUMA, GPU, IB)
• src/modules/hpc/healthcheck.rs — HPC health validation
• src/modules/hpc/mpi.rs — MPI library config (OpenMPI, Intel MPI)
• src/modules/hpc/munge.rs — MUNGE authentication (Slurm prerequisite)
• src/modules/hpc/nfs.rs — NFS server/client management
• src/modules/hpc/slurm.rs — Slurm controller/compute/ops
• src/modules/hpc/toolchain.rs — Development toolchain setup
• src/modules/hpc/partition_policy.rs — Partition policy helper types

Acceptance criteria

• Each module has at least 3 unit tests covering parameter parsing, state validation, and module metadata
• All tests pass with cargo test --lib --features full-hpc

Priority

Medium — implementations are complete and functional, tests needed before GA