KW becomes levels() (scalar L1 + scalar L2, per-level blocks, objects,
Jets). All rejection cases are kept per level. Two cases change branch:
L2 bodies/Jets with no nested child (legacy 'child_objects/child_jets
require a nested child') are now an entry for a level that does not
exist ('needs one entry per level'); several L1 patches + scalar child
now meets 'a scalar child requires a scalar parent'. Both still reject
before CUDA. Assertions unchanged.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>