Merge: collection ordering, and a range that says where it ends

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

# Conflicts:
#	docs/traceability.md
#	ui/dr-ui/src/collections_ui.rs
#	ui/dr-ui/src/library.rs
#	ui/dr-ui/ui/app.slint
#	ui/dr-ui/ui/library.slint
#	ui/dr-ui/ui/widgets.slint

core/dr-catalog/src/collections.rs

@@ -376,6 +376,64 @@ pub fn set_order(
     touch(conn, id)
 }
 
+/// Whether this collection has a manual order worth reading.
+///
+/// The grid asks before choosing an `ORDER BY`, and the answer is narrower
+/// than "is it manual". Two things have to be true.
+///
+/// It must be **manual**: a saved filter's contents are whatever its selector
+/// matches now, in whatever order the query returns them, and there are no
+/// member rows to carry a position.
+///
+/// And it must have **no children**. A parent's contents are the union of its
+/// own members and its descendants' ([`descendants`]), and positions are only
+/// ever assigned within one collection — so two children's positions are
+/// unrelated integers, and interleaving them by value would order the grid by
+/// a coincidence. Capture time is the honest answer for a set.
+///
+/// Kept here rather than assembled at the call site, so the rule has one name
+/// and one test, and the grid does not have to know that "no children" is part
+/// of it.
+pub fn orders_manually(conn: &Connection, id: CollectionId) -> Result<bool, CatalogError> {
+    if kind_of(conn, id)? != CollectionKind::Manual {
+        return Ok(false);
+    }
+
+    let has_child: Option<i64> = conn
+        .query_row(
+            "SELECT 1 FROM collections
+              WHERE parent_id = ?1 AND deleted = 0 LIMIT 1",
+            [id.0 as i64],
+            |r| r.get(0),
+        )
+        .optional()?;
+
+    Ok(has_child.is_none())
+}
+
+/// Every image in a manual collection, in manual order.
+///
+/// The whole membership, not the filtered view the grid happens to be showing.
+/// [`set_order`] renumbers exactly the images it is handed, so a caller that
+/// reordered a filtered list would renumber those and leave every hidden image
+/// on a stale position — two images sharing a position, and a grid that
+/// rearranges itself when the filter comes off.
+///
+/// `image_id` breaks ties. Positions are dense in practice, but a merge can
+/// deliver two devices' rows at the same position, and an order that is not
+/// total is one the grid draws differently each time it reads it.
+pub fn members_in_order(conn: &Connection, id: CollectionId) -> Result<Vec<ImageId>, CatalogError> {
+    let mut stmt = conn.prepare(
+        "SELECT image_id FROM collection_members
+          WHERE collection_id = ?1
+          ORDER BY position, image_id",
+    )?;
+    let rows = stmt
+        .query_map([id.0 as i64], |r| Ok(ImageId(r.get::<_, i64>(0)? as u64)))?
+        .collect::<Result<Vec<_>, _>>()?;
+    Ok(rows)
+}
+
 /// Save a smart collection's selector.
 ///
 /// Refuses a selector that references this collection, directly or through
@@ -789,6 +847,106 @@ mod tests {
         ImageId(i)
     }
 
+    #[test]
+    fn a_manual_leaf_orders_manually() {
+        let cat = seeded();
+        let c = cat.connection();
+        let id = create(c, "Trip", None, CollectionKind::Manual).unwrap();
+        assert!(orders_manually(c, id).unwrap());
+    }
+
+    #[test]
+    fn a_saved_filter_has_no_manual_order() {
+        let cat = seeded();
+        let c = cat.connection();
+        let id = create(c, "Picks", None, CollectionKind::Smart).unwrap();
+        assert!(
+            !orders_manually(c, id).unwrap(),
+            "a smart collection has no member rows to carry a position"
+        );
+    }
+
+    #[test]
+    fn a_parent_has_no_manual_order_of_its_own() {
+        let cat = seeded();
+        let c = cat.connection();
+        let parent = create(c, "2026", None, CollectionKind::Manual).unwrap();
+        let child = create(c, "Iceland", Some(parent), CollectionKind::Manual).unwrap();
+
+        assert!(
+            !orders_manually(c, parent).unwrap(),
+            "a set shows its descendants' images, whose positions are unrelated"
+        );
+        assert!(
+            orders_manually(c, child).unwrap(),
+            "the child is still a leaf and still orders manually"
+        );
+    }
+
+    #[test]
+    fn deleting_the_last_child_gives_the_parent_its_order_back() {
+        let cat = seeded();
+        let c = cat.connection();
+        let parent = create(c, "2026", None, CollectionKind::Manual).unwrap();
+        let child = create(c, "Iceland", Some(parent), CollectionKind::Manual).unwrap();
+        assert!(!orders_manually(c, parent).unwrap());
+
+        delete(c, child).unwrap();
+        assert!(
+            orders_manually(c, parent).unwrap(),
+            "a tombstoned child must not keep counting as a child"
+        );
+    }
+
+    #[test]
+    fn members_come_back_in_the_order_they_were_put_in() {
+        let cat = seeded();
+        let c = cat.connection();
+        let id = create(c, "Trip", None, CollectionKind::Manual).unwrap();
+        add_images(c, id, &[img(3), img(1), img(2)]).unwrap();
+
+        assert_eq!(
+            members_in_order(c, id).unwrap(),
+            vec![img(3), img(1), img(2)],
+            "adding assigns position in the order given, and reading returns it"
+        );
+    }
+
+    #[test]
+    fn members_come_back_in_the_order_set_order_wrote() {
+        let cat = seeded();
+        let c = cat.connection();
+        let id = create(c, "Trip", None, CollectionKind::Manual).unwrap();
+        add_images(c, id, &[img(1), img(2), img(3)]).unwrap();
+
+        set_order(c, id, &[img(3), img(2), img(1)]).unwrap();
+        assert_eq!(
+            members_in_order(c, id).unwrap(),
+            vec![img(3), img(2), img(1)]
+        );
+    }
+
+    #[test]
+    fn members_in_order_is_total_even_where_positions_collide() {
+        let cat = seeded();
+        let c = cat.connection();
+        let id = create(c, "Trip", None, CollectionKind::Manual).unwrap();
+        add_images(c, id, &[img(1), img(2), img(3)]).unwrap();
+
+        // What a merge can deliver: two devices' rows landing on one position.
+        c.execute(
+            "UPDATE collection_members SET position = 0 WHERE collection_id = ?1",
+            [id.0 as i64],
+        )
+        .unwrap();
+
+        assert_eq!(
+            members_in_order(c, id).unwrap(),
+            vec![img(1), img(2), img(3)],
+            "image_id breaks the tie, so two reads cannot disagree"
+        );
+    }
+
     #[test]
     fn a_created_collection_appears_in_the_tree() {
         let cat = seeded();