[HOT FIX] Stabilize macOS CI tests: OpenMP linking, MPI env, per-test isolation

[allnes]

• macOS OpenMP: use OpenMP::OpenMP_CXX, detect Homebrew libomp (include/lib), and
  add compiler flags so AppleClang actually enables OpenMP.
• MPI runs: propagate PPC_NUM_THREADS and OMP_NUM_THREADS via mpirun -x …; fix
  gtest filters for all/mpi across functional/perf runs.
• Per-test isolation: add ScopedPerTestEnv that sets PPC_TEST_UID and
  PPC_TEST_TMPDIR; integrated into functional and performance test runners.

[codecov]

Codecov Report

❌ Patch coverage is 91.66667% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 94.58%. Comparing base (c83f0ef) to head (461bf7c).

Files with missing lines	Patch %	Lines
modules/util/include/util.hpp	91.66%	0 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #669      +/-   ##
==========================================
- Coverage   94.73%   94.58%   -0.16%     
==========================================
  Files          15       15              
  Lines         456      480      +24     
  Branches      166      177      +11     
==========================================
+ Hits          432      454      +22     
- Partials       24       26       +2     

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