Reconcile what four branches each built separately

Three seams, found by the first compile after the merge.

Two branches implemented 'can this scope be reordered' independently:
one from the collections model, one from the catalog through
orders_manually, on every re-read and excluding the trash. The second is
the better answer and is what survives; it only needed to set the
property app.slint declares.

Two lints from scene-mask-ui, which was merged mid-flight and had never
been through -D warnings: an is_none check spelled out where clippy wants
?, and a return in a cfg block's tail.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

ui/dr-ui/src/develop.rs

@@ -2648,9 +2648,7 @@ impl DevelopSession {
         // Refuse a category this run did not produce rather than creating a
         // layer that renders empty: an empty mask looks like a broken
         // adjustment, where a button that does nothing at least says so.
-        if seg.category_mask(name).is_none() {
-            return None;
-        }
+        seg.category_mask(name)?;
 
         let id = self.graph.masks().next_id();
         let mut layer = MaskLayer::new(