Fix DG1 XDMF export with element-local simplex topology

[gthyagi]

Problem

DG1 fields were treated as one-value-per-cell XDMF attributes even though triangles have three and tetrahedra four local coefficients. This creates invalid visualization metadata and cannot preserve the discontinuous linear field correctly.

Changes

• Add a disconnected DG1 grid alongside the standard mesh grid in one XDMF file.
• Evaluate each cell polynomial at its own physical vertices using native cell maps; no coordinate matching or averaging across cells.
• Write only owned cells through PETSc collective output, including ranks with no owned cells.
• Support scalar, vector, full tensor and symmetric tensor fields on full-dimensional triangles and tetrahedra.
• Preserve native HDF5 checkpoint/reload data. Unsupported DG visualization fails explicitly; native-only output remains available.
• Document the layout and ParaView usage.

Verification

• Rebuilt UW3 in amr-dev.
• 24 focused serial tests passed (DG1, existing XDMF compatibility and save/load).
• All four DG1 regression tests passed on eight MPI ranks.
• ParaView 6.0 read both 2D and 3D outputs and rendered the 2D field; maximum scalar errors were 1.78e-15 and 3.55e-15 respectively.
• Regression tests cover preserved jumps, physical vertex values, positive cell orientation, ownership, tensor packing, native checkpoint reload and unsupported layouts.
• Deprecated-pattern check and git diff --check passed.

This changes visualization output only; it does not change the Stokes solver or smooth stress/strain-rate fields.