KW (legacy base/fine/child blocks + child_rectangles_in_parent) becomes
levels(): refinements=[PARENTS, CHILDREN], level_blocks, level_objects,
level_jets. Every negative case is kept and expressed per level (child
rectangles, per-level blocks); the L1-body case now expects the level-list
rule's message 'only on the finest level' instead of the legacy keyword
message 'only on L2' (same constraint).
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>