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/develop/masks.rs

@@ -543,7 +543,9 @@ impl DevelopSession {
             .record(&self.graph, Edit::Action(labels::step::MASK_PART_REMOVED));
     }
 
-    /// Change how a part joins: added to the mask, or taken out of it.
+    /// TRACES: FR-DEV-19a
+    /// Change how a part joins: added to the mask, taken out of it, or kept
+    /// only where the mask already was. `join` indexes `Join::ALL`.
     pub fn set_mask_part_join(&mut self, id: &str, index: usize, join: usize) {
         use dr_pipeline::mask::Join;
         let Some(&join) = Join::ALL.get(join) else {