Per clarification: the 721 group lines are clip markers, NOT segments. Segments come
ONLY from user split points, numbered globally from 1 — so the first split is seg1, not
seg23. 0 splits = 0 segments; N splits = N segments (seg k = point k-1 .. split k, tail
after the last split excluded). group lines still drawn (bold) + used for the group#.
JS matches (segPoints/nSegs). Empty summary now prompts to add splits.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>