On a parent page a running-header variable shows <Name>, as InDesign shows it (it was blank). Text
with no page that isn't on a parent (a frame composition can't place on a page) keeps the
variable's text before and after: frames now record whether they are on a parent spread, and
the composed values are cached by page and parent.
Split from #1232 (parent-page-display), as asked in review.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01CgGsJkVe5GYG9qK6VFdECa