Split a chunk into completed / band / ghost draw ranges around a progress position
(DD-006 §4.5): the completed cut sits at loSeg, the ghost starts after hiSeg,
and the emphasis band covers the uncertainty between them. Same O(log n) bounds as
computeDrawState; composes with the layer range by intersection. Pass loSeg = -1
for "nothing completed yet" (everything after the band is ghost).
Split a chunk into completed / band / ghost draw ranges around a progress position (DD-006 §4.5): the completed cut sits at
loSeg, the ghost starts afterhiSeg, and the emphasis band covers the uncertainty between them. Same O(log n) bounds ascomputeDrawState; composes with the layer range by intersection. PassloSeg = -1for "nothing completed yet" (everything after the band is ghost).