Segfault on imported DMPlex meshes with essential BCs

[jcgraciosa]

`tests/test_1012_stokesImportedDMPlex.py::test_stokes_essential_bc_imported_dmplex` segfaults (SIGSEGV, exit 139) as of 2026-06-24.

Confirmed universal across 3 independent builds (release `amr-dev` petsc4py 3.25.2, daily-dev `amr-dev` petsc4py 3.25.0, plain `dev` petsc4py 3.24.4) — not AMR-specific, not a build artifact.

This is the regression test for PR #86 (`gthyagi/codex/fix_stokes_essential_bc_imported_dmplex`), merged 2026-03-20 — after `v3.0.1`. Repro: gmsh mesh → write to file → `PETSc.DMPlex().createFromFile()` → `uw.discretisation.Mesh` wrapping with `markVertices=True, useMultipleTags=True, useRegions=True` → Stokes LU solve. Crashes as a hard segfault, no Python traceback.

Currently skipped in `release/v3.1.0` to unblock v3.1.0 validation. Needs a gdb/lldb backtrace to actually triage.

[lmoresi]

Fixed by the if label_is: truthiness guard in _dm_unstack_bcs() — landed on release/v3.1.0 (#287) and development (#288). Avoids the PETSc IS getSize() error on imported DMPlex meshes carrying non-boundary (e.g. Elements/cell-region) labels.