Track Ubuntu 24.04 MPICH pFUnit re-enable path

[jaharris87]

Context

Issue #255 / PR #257 root-caused the MPICH pFUnit PE=0 failure on hosted runners to the Ubuntu 24.04 apt MPICH/Hydra packaging bug: /usr/bin/mpiexec.mpich -n 2 can launch two singleton MPI_COMM_WORLD processes instead of one two-rank world. pFUnit then reports Insufficient processes to run this test. (PE=0) for [npes=2] tests because each process sees only one PE.

PR #257 keeps MPICH smoke/install-consumer and MPICH pFUnit coverage on hosted Ubuntu 22.04, with a raw MPI_COMM_WORLD size probe before the jobs claim coverage. This is intentionally conservative until the hosted Ubuntu 24.04 apt package path is fixed.

Relevant external tracker: https://bugs.launchpad.net/ubuntu/+source/mpich/+bug/2072338

Goal

Move the MPICH CI jobs back to ubuntu-latest / Ubuntu 24.04 once the hosted-runner distro package path reliably launches multi-rank MPICH jobs.

Acceptance criteria

• Confirm the current GitHub-hosted ubuntu-latest image and apt mpich / libmpich-dev versions.
• Confirm /usr/bin/mpiexec.mpich -n 2 runs a raw MPI probe with ranks 0 and 1, both seeing MPI_COMM_WORLD size 2.
• Confirm the MPICH pFUnit job builds pFUnit with the same MPICH wrapper used by fTimer.
• Confirm both ftimer_mpi_tests and ftimer_mpi_tests_4pe pass under /usr/bin/mpiexec.mpich.
• If all of the above pass, move the MPICH CI jobs from ubuntu-22.04 back to the preferred current hosted Ubuntu runner and update docs/comments accordingly.

Notes

Do not remove the raw launcher probe when making this change; it protects both pFUnit and the smoke/install-consumer checks from false-positive singleton-world launches. Do not broaden MPICH pFUnit support claims until CI validates the fixed hosted-runner path.

[jaharris87]

Mapped into the v1.0 readiness umbrella #348 as V1-NB-03: Ubuntu 24.04 MPICH pFUnit runner migration, and cross-linked from #353 for the MPICH MPI+OpenMP evidence-boundary decision.

Disposition for v1.0 planning: this remains non-blocking runner maintenance unless maintainers decide that moving MPICH jobs back to current hosted Ubuntu is required for the v1.0 support claim. The v1.0 blocker is #353, which decides whether MPICH MPI+OpenMP hybrid evidence remains local-only/caveated or becomes permanent CI coverage.