The tree threads no longer touch their arrows. Each thread is
BRACKETED by checkbox bottom edges (live-measured anchors):
- starts at its parent row's checkbox bottom edge — a visible gap
below the arrow's stroke (the stroke ends ~2.4px above the anchor)
- terminates exactly at the LAST child row's checkbox bottom edge
(cluster: top-[23px]/bottom-[8px] incl. the 1px group rule; chain:
top-[22px]/bottom-[8px]). Verified live: 23=23 top anchor, bottom
ends within 0.13px of the last checkbox at both levels.