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 <[email protected]>

src/app/mod.rs

@@ -3854,6 +3854,9 @@ pub enum Message {
     AttachPick,
     /// DWFATTACH / DGNATTACH: pick the file (the result goes the ATTACH way).
     UnderlayAttachPick(codec::entities::UnderlayType),
+    /// POINTCLOUDATTACH: pick the scan or project (the result goes the
+    /// ATTACH way).
+    PointCloudAttachPick,
     /// The Reference slide-out's xref fading: amount dragged, drag done,
     /// switch.
     XrefFadeSlide(u8),