The zones already imported, rendered and exported per section, but only
section 1 was editable state — the rest were marked read-only.
Editing now targets the section of the page it starts on: a double-click
edits the zone under the pointer, the Layout-panel buttons the current
page's. zoneKey/writeZone route the single live editor's read and its
write-back to either the six per-zone props or the extraHfSections entry.