Replace a single section by heading

Targeted-edit counterpart to PUT /doc (full replacement) and POST /doc/sections (append). Finds the FIRST heading whose plain text matches heading (case-sensitive, trimmed), then replaces everything from that heading up to the next heading at the same or shallower level. markdown is the FULL replacement INCLUDING the heading line; pass an empty string to delete the section. Returns 404 when no matching heading exists, by design, so a misremembered heading fails loudly. Same byte/depth/node-count guard as PUT /doc; same writeDocBody attribution path.