Preserve Cartesian vacuum query coordinates and mark unavailable stencils #293

Cartesian export can label a query with its original coordinates after moving its field evaluation to a source sample or downcasting it.

Expected behavior: Opt Cartesian export into exact double queries, skip unresolved source-crossing stencils before Green evaluation, and guard requested nonlocal processing of unavailable fields.

Implementation scope: vacuum_ma pickup/MSCFLD/CHI; gpout Cartesian calls; portable tests. Keep this as one independently reviewable fix or feature against the repository main development branch.

Verification: cd tests; FO_JOBS=2 OMP_NUM_THREADS=1 OPENBLAS_NUM_THREADS=1 fo.

Acceptance evidence: Independent direct CHI, analytic complex harmonic, active closed wall, legacy byte identity, exact masks and specific postprocessor rejections; source quadrature/physical forcing remain separate.

Use independent behavioral references; a repository-state or patch-equality check alone is insufficient. The linked PR will record exact source, review order, completed checks and any draft work.

Implementation: PR294. Reviewable implementation; the PR records completed validation and scope.

Chris&AI