Follow-ups from #218: route snapshot subsystem off deprecated write_checkpoint + XDMF cleanups

[lmoresi]

Non-blocking follow-ups identified while reviewing #218 (PETSc-native XDMF viz topology, merged bc834b4):

1. write_checkpoint() FutureWarning fires from UW3's own snapshot subsystem. src/underworld3/checkpoint/disk_snapshot.py:314 calls mesh.write_checkpoint(...), which Use PETSc-native XDMF visualization topology #218 deprecated. Harmless today (setup.cfg filterwarnings has no error escalation) but self-inflicted warning spam. Fix: have the snapshot backend call _write_petsc_reload_file / write_timestep(..., petsc_reload=True) directly, or suppress the warning for internal callers.

2. Dead/fragile attr read in checkpoint_xdmf: topo["cells"].attrs["cell_dim"] is read but never used (topology type is derived from numCorners+spaceDim). Delete it — it's a needless dependency on a PETSc-internal attr name that could KeyError on a future PETSc/cell type.

3. Unify XDMF attribute writers: Use PETSc-native XDMF visualization topology #218 flattened the meshVar attribute DataItem (removed HyperSlab) but the parallel swarmVar block still uses ItemType="HyperSlab". Not a regression; unify for consistency.

cc @gthyagi

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[lmoresi]

Items 1 (snapshot FutureWarning) and 2 (dead cell_dim read) resolved by #256 (merged to development).

Item 3 remains open: unify the swarmVar XDMF attribute block (still ItemType="HyperSlab") with the flattened meshVar form from #218. Deferred as cosmetic + ParaView-sensitive (no easy local render validation). Lower priority.