Read-side companion to doc-find-deep + reparent: ancestor-section chain
(root-first) for a block id, nil if absent (distinct from () top-level path);
block-depth is the path length. For breadcrumbs / scoping. New suite +13.
Probe this pass found no bugs: clone/remap tree-wide, all block types have
asMarkdown:.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
