test: expand surgical coverage to 50 cases

Add 31 tests across editNodeText edge cases (empty rename, inline
markdown, fold-suffix preservation, H6 clamp, multi-line continuation),
removeNodes batch (non-adjacent siblings, last-child, full section),
insertChild variants (empty root, folded parent, ordered marker),
insertSibling edge cases (folded ref, end-of-root, ordered list),
insertParent edge cases (H6 clamp, deep list, intact code block),
moveBlock kind×kind matrix (heading/heading, list/list deep, list/
heading roundtrip), setFold idempotency, and insertSubtree variants
(deep copy, heading→list conversion, fold preservation).

50 / 50 passing.

Co-Authored-By: Claude <noreply@anthropic.com>
Signed-off-by: Andrei Kvapil <kvapss@gmail.com>

Author: kvaps