`link_list` and `form_set_props` indexed the viewer's pages with a page number from the
editing layer. On a file whose page tree has a dangling `/Kids` entry (#2660) the viewer
sees three pages and the editing layer six, so `link_list` panicked ("the len is 3 but
the index is 3") and so did moving a field on one of the later pages.
Rectangles on a page the viewer could not read are now reported unchanged, as `doc_info`
already reports them (`link_list`, `comment_list`, `content_list`); `comment_list` no
longer borrows the last page's geometry for them. Placing a rect on such a page is an
error (`form_set_props`, and `sign` when the viewer has no page at all).
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>