Offer a mask that is nothing but a hand

Every route to a layer began with a selection — a gradient, a band, a subject,
a category — and painting was reachable only by making one of those and
joining a painted part to it. So the answer to "brush a correction onto this
corner of the sky" was "add a radial gradient you do not want, then paint into
that", which is not an answer.

Paint sits beside Linear and Radial and makes a layer whose base is a brush.
It covers nothing until a stroke lands in it, so pressing it arms the brush
and shows the mask as well: a row that appeared and changed no pixel, with the
pointer still in "select", is indistinguishable from a button that did nothing.

ui/dr-ui/src/masks_ui.rs

@@ -1010,6 +1010,37 @@ pub(crate) fn wire(
             redraw(&w);
         });
     }
+    // TRACES: FR-DEV-19b
+    {
+        let weak = window.as_weak();
+        let session = session.clone();
+        let redraw = redraw.clone();
+        let rows = rows.clone();
+        window.global::<Masking>().on_add_brush(move || {
+            let Some(w) = weak.upgrade() else { return };
+            let made = session
+                .borrow_mut()
+                .as_mut()
+                .and_then(|s| s.add_brush_mask())
+                .is_some();
+            // Armed, and this is not a convenience: the layer covers no pixel
+            // until something is painted into it, so a press that made a row
+            // and left the pointer in "select" would be indistinguishable from
+            // a button that did nothing. The same reasoning as `on_part_added`
+            // below, and the tool's own handler turns the mask view on.
+            if made {
+                w.global::<Masking>().set_tool(1);
+                if let Some(s) = session.borrow_mut().as_mut() {
+                    if s.mask_view() == 0 {
+                        s.set_mask_view(1);
+                    }
+                }
+            }
+            sync(&w, &session);
+            sync_rows(&w, &rows, &session);
+            redraw(&w);
+        });
+    }
     // TRACES: FR-DEV-10
     {
         let weak = window.as_weak();