Kernel rig: import/4561 passes, off the known-failing list

The zero-area triangle is dropped (previous commit): the box converts with six
faces and V = 1000 mm^3, as the case expects.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/kernel-cases/import/KNOWN_FAILING

@@ -2,4 +2,3 @@
 # rig gate (.github/workflows/kernel-rig.yml) fails on a case that fails and is not listed here,
 # and on a listed case that passes: a fix removes its line in the same pull request.
 # Baseline: develop 5ada4fc8a with SMLib 90aabff (the run on 60b1c6a, aarch64 Linux).
-4561-convert-box-mesh-with-a-zero-area-triangle step 3 (mesh.convert): a zero-area triangle is refused as degenerate where the case expects it dropped