A moved comment is drawn where it goes while it is dragged

Moving a rectangle, an oval or any other comment showed only its blue
frame going, the shape staying where it was until release. The comment is
now rendered alone once as the drag starts (Document::comment_layer: a
read-only copy with only that comment, no page content) and laid over the
page where it goes; the page beneath keeps showing it where it was until
the move is applied, as a CAD program's move does. Rendering the page
without it would cost a whole page's render at every drag on a heavy plan.
Stamps and Fill & Sign marks keep their own layers. annot: isolate_annotations
for several comments of a page, so a group can be drawn together.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/ui-parity.md

@@ -47,6 +47,7 @@ No dump of Acrobat's menu tree has been diffed against the registry yet (gap 24)
 | Hand tool, middle-button pan, autoscroll | yes | hand tool; Linux autoscroll | partial |
 | Text selection, column select | yes, Alt-drag columns | reading-order selection; Alt/Option-drag or Edit ▸ Column select (#740) | shipped |
 | Select and move several objects at once | yes | on one page: Shift/Ctrl/⌘-click and marquee, one undo step (#844); not across pages, no group resize | partial |
+| Moved comments drawn where they go while dragged | yes | yes: the comment's own pixels follow the pointer, the page shows it where it was until release; stamps and Fill & Sign marks also hide the original | shipped |
 | Object handles: corners keep proportion, edges free | yes | yes for added images and signatures; Shift on a corner keeps the proportions of comments, stamps and form fields (#2199); no Alt resize from the centre | shipped |
 | Arrange, align and distribute page objects | yes | fields only | partial |
 | Rulers, grids, snap to grid, guides | yes | no | planned |