Point clouds: drawn from their scans, placed, cropped and attached as the reference does

Scans (.rcs) and projects (.rcp) are read and their points drawn as
screen-sized dots, coloured by the cloud's stylization. The cloud's
placement carries its scale and rotation; its extents and crops stay in
the scan's coordinates. A missing scan shows the extents with the saved
path. -POINTCLOUDATTACH / POINTCLOUDATTACH, POINTCLOUDCROP and the
POINTCLOUD* settings follow the reference's prompts and messages, the
Properties list is the reference's, and a locked cloud is left alone by
edits.

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

src/app/update/mod.rs

@@ -1429,6 +1429,7 @@ impl OpenCADStudio {
             }
             message @ (Message::AttachPick
             | Message::UnderlayAttachPick(_)
+            | Message::PointCloudAttachPick
             | Message::AttachPickResult(_)
             | Message::XrefAttach(_)
             | Message::XrefAttachBrowseResult(_)) => self.update_xref_attach(message),