Toggling/cycling/rejecting a nested checkbox now recomputes each
ancestor's progress marker (vimwiki's listsyms_propagate). Mirrors
vimwiki's averaging — rejected items count as 100% unless every child
is rejected, in which case the parent itself becomes rejected. Walks
up while each ancestor has its own checkbox.
Also tightens find_checkbox_span to scan only the marker line, since a
parent item's span covers its nested sublist and was matching child
markers as if they belonged to the parent.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
gffranco
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