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/develop.rs

@@ -3434,6 +3434,37 @@ impl DevelopSession {
         Some(id)
     }
 
+    /// TRACES: FR-DEV-19b
+    /// Add a mask that is nothing but hand-painted, and select it.
+    ///
+    /// **The one route to a brush that starts from nothing.** Everything else
+    /// in this file makes a layer out of a selection — a gradient, a band, a
+    /// subject, a category — and painting was reachable only by making one of
+    /// those first and then 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 it", which is not an answer.
+    ///
+    /// The layer covers nothing until a stroke lands in it, which is exactly
+    /// what [`dr_pipeline::mask::MaskPart::covers`] is about: it is not active,
+    /// it costs no slice, and an invert on it would not take the adjustment
+    /// global. What the panel shows meanwhile is a row with the tools armed
+    /// over it — see `masks_ui`, which arms them.
+    pub fn add_brush_mask(&mut self) -> Option<String> {
+        let id = self.graph.masks().next_id();
+        if !self
+            .graph
+            .masks_mut()
+            .push(MaskLayer::new(id.clone(), MaskSource::brush()))
+        {
+            return None;
+        }
+        self.active_masks = vec![id.clone()];
+        self.active_part = 0;
+        self.history
+            .record(&self.graph, Edit::Action(labels::step::MASK_ADDED));
+        Some(id)
+    }
+
     /// TRACES: FR-DEV-10
     /// Add a mask that selects by a range of the photograph's own values.
     ///