Resolve a plan node's body — its prose sections, sorted by order key. Returns { sections: [{ id, text, orderKey }] }; a node with no body returns an empty list.

Replace a plan node's whole body with the given sections, in display order. The substrate mints fresh order keys and copy-on-writes changed prose. Returns { ok, count }.

Append one new section to the end of a plan node's body. Returns the appended { section } and the new body { count }.

Replace the prose of a single section (copy-on-write), keeping its position and every other section untouched. Returns the edited { section }.