Rank 0 CXUs are set-scoped CXUs that are always included, determined by `is_rank0` on set membership, so that certain context units are guaranteed to be in scope.

**Rank 0 CXUs:** Set-scoped always-included CXUs (is_rank0 on set membership)

— PYRANA_PLATFORM_ARCHITECTURE_REFERENCE.md

Set-scoped always-included CXUs

— PYRANA_PLATFORM_ARCHITECTURE_REFERENCE.md

is_rank0 on set membership

— PYRANA_PLATFORM_ARCHITECTURE_REFERENCE.md