Point clouds: attach dialog, crop toolbar, stylize, palette rows

POINTCLOUDATTACH opens the Attach Point Cloud dialog after the file is
chosen; its Attach button lives in the Attach Underlay list instead of a
separate Insert panel. A selected cloud brings the crop tools to the
right-edge toolbar. POINTCLOUDSTYLIZE sets the stylization with the
reference's prompts, the External References palette lists point clouds
(unload, reload, detach, path), and definitions keep the header extents.

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

src/io/xref_model.rs

@@ -131,7 +131,7 @@ fn root_key(normalized: &str) -> String {
     String::new()
 }
 
-pub fn is_relative_path(normalized: &str) -> bool {
+fn is_relative_path(normalized: &str) -> bool {
     let b = normalized.as_bytes();
     if b.len() >= 2 && b[0].is_ascii_alphabetic() && b[1] == b':' {
         return false;
@@ -305,6 +305,8 @@ pub enum RefKind {
     Image,
     /// A PDF, DWF or DGN underlay.
     Underlay,
+    /// A point cloud scan (.rcs) or project (.rcp).
+    PointCloud,
 }
 
 /// Lifecycle state of a [`ReferenceEntry`].