[codex] Add COMSOL offline postprocessing guidance

[jiweiqi]

Summary

Adds optional COMSOL skill guidance for Python-friendly/offline postprocessing exports after a solve.

• Links a new optional reference from the COMSOL skill index and offline section.
• Documents a reusable bundle with saved .mph, manifest.json, mph_summary.json, VTU fields, and CSV tables.
• Adds headless JPype/Java API snippets for volume VTU, surface VTU, and table CSV export.
• Keeps this guidance opt-in: no required checkpoint rule, no driver behavior change, no plugin-index/release/public-doc changes.

Real COMSOL Validation

Ran a headless COMSOL 6.4.0.293 smoke using the bundled block_with_hole heat-transfer workflow:

• sim check comsol with UTF-8 forced: COMSOL 6.4 detected.
• sim connect --solver comsol --ui-mode no_gui.
• Executed geometry, material, physics, mesh, solve, and plot steps successfully.
• Exported a validation bundle under a temp workdir:
  • saved .mph
  • exports/manifest.json
  • exports/mph_summary.json
  • exports/fields/domain_solution.vtu
  • exports/fields/boundary_fields.vtu
  • exports/tables/global_metrics.csv

Offline checks after export:

• inspect_mph read the saved .mph as node_type=solved, comsol_version=6.4.0.293, physics_tags=["ht"], material_tags=["mat1"].
• meshio opened both VTUs:
  • domain VTU: 926 points, 3908 tetra cells, point data Temperature
  • boundary VTU: 498 points, 996 triangle cells, point data Temperature
• After stopping COMSOL, plain Python/stdlib parsed the VTU XML and CSV.
• CSV metrics exported from COMSOL table:
  • average T: 326.19111807164984 K
  • max T: 373.0000000000001 K
  • min T: 292.9999999999999 K

Other Checks

• git diff --cached --check
• local markdown link sanity check for changed files

Notes

The real validation found one useful table-export gotcha: COMSOL can write only table metadata if a derived-value table is not populated first or the numerical selection is empty. The reference now calls that out explicitly.