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/scene/mod.rs

@@ -13924,6 +13924,14 @@ mod update_entities_batch_tests {
     }
 }
 
+pub(crate) fn is_point_cloud(entity: &EntityType) -> bool {
+    matches!(
+        entity,
+        EntityType::Extended(extended)
+            if matches!(extended.data, codec::entities::ExtendedEntityData::PointCloudEx(_))
+    )
+}
+
 /// A point cloud whose Locked property is set: edits leave it alone.
 pub(crate) fn is_locked_point_cloud(entity: &EntityType) -> bool {
     matches!(