CI: add mpi/distributed module to free CI pipe - #814

Adds three free CI jobs, all using minidmr, an open-source tool built by our team that allows creating a reproducible HPC cluster on Docker to simulate a multi-node MPI cluster . minidmr is already integrated into the CI pipelines of two of our other projects, and is also used to run MPI tests locally.

By using it on QuEST, this lets your CI pipeline run MPI tests on a single free ubuntu-latest runner. test_paid.yml is untouched for now; however, after a careful review, it could be removed in a follow-up to save money.

This PR adds:

1. test_free.yml: distributed-minidmr-test: full functional tests over real MPI (minidmr, 4 simulated nodes).
2. audit.yml: sanitisation-test-mpi: ASan over comm/* (the MPI code), minidmr with 2 nodes : the first time this code has run under a sanitiser.
3. audit.yml: coverage-test-mpi: lcov over comm/* via minidmr, 2 nodes: complements the existing serial coverage job, which can't exercise MPI.

Impact:

• Cost: replaces the paid equivalent (mpi=ON, cuda=OFF in test_paid.yml, ~$4.80/run, only triggerable manually via workflow_dispatch) with free jobs on every push/PR, saving ~$5/run.
• Coverage: comm/* goes from 0% measured (explicitly excluded from the serial coverage job, which doesn't compile with MPI) to comm/comm_config.cpp at 77.4% and comm/comm_routines.cpp at 4.3% line coverage.