Slabbed

Hotfix on 0.5.2-alpha.2:

A chain hanging below an ordinary (non-slab) lowered block now follows that block's exact drop, matching what already hung correctly further down the same chain. Previously the chain segment itself stayed at grid height, so the lowered block above visibly descended into it. A chain under a lowered TOP/DOUBLE slab is unaffected: that case keeps its own dedicated flush treatment.