Offer intersection in the mask panel: an Intersect button and a third chip state

The pipeline could now keep only where two selections agree, but the panel
had no way to ask for it: the part row's chip flipped between + and -, and
the buttons under the parts joined an added or a subtracted correction.

An "∩ Intersect" button joins a painted part that intersects, and the chip
on a part row cycles + -> - -> ∩ and round, so an existing part can be
turned into an intersection without being repainted. Both go through the
same session calls as before, indexing Join::ALL, whose first two entries
kept their places. The chip is now a tagged gesture, so it is in the
gesture book.

ui/dr-ui/src/masks_ui.rs

@@ -1534,6 +1534,31 @@ mod tests {
         assert_eq!(s.mask_feather(&id), 0.03, "and the base was left alone");
     }
 
+    /// TRACES: FR-DEV-19a
+    /// The panel's third join reaches the layer: the "∩ Intersect" button
+    /// joins an intersecting part, the part row reports it as the chip's third
+    /// state, and cycling the chip on from there comes back round to a union.
+    #[test]
+    fn the_intersect_button_joins_a_part_that_intersects() {
+        use dr_pipeline::mask::Join;
+        let Some(mut s) = session() else {
+            eprintln!("no adapter; skipping");
+            return;
+        };
+        let id = s.add_gradient_mask(true).expect("radial");
+        s.set_active_mask(Some(&id));
+
+        let index = s.add_mask_part(&id, 2).expect("a part was joined");
+        let join = |s: &DevelopSession| {
+            s.masks().get(&id).expect("layer").part(index).expect("part").join
+        };
+        assert_eq!(join(&s), Join::Intersect);
+        assert_eq!(s.mask_parts(&id)[index].2, 2, "the chip shows ∩");
+
+        s.set_mask_part_join(&id, index, (2 + 1) % 3);
+        assert_eq!(join(&s), Join::Union, "and the chip cycles back to +");
+    }
+
     /// TRACES: FR-DEV-3 | FR-UI-1
     /// The fault this pass exists for, stated as a test.
     ///