Show the regions, click one, and adjust it

The local panel sits above the adjust panel because it decides what those
sliders act on; below it, a photographer would set an exposure and only
then discover which scope it landed in. Selecting a layer re-scopes the
existing controls to that layer's chain — there is no second set of
sliders, and there must not be, or every operation added to `ops/` would
need a local twin.

The overlay is drawn over the canvas rather than blended into the render,
because it is a diagnostic and not an edit: it must not reach the
histogram, an export, or the texture handed to the compositor. Nearest-
neighbour always — the map's values are *names*, so smoothing between
region 4 and region 9 invents a colour belonging to neither and softens
exactly the edge the overlay exists to show.

Picking gets its own touch area above the pan handler. Panning wants
press-drag-release and picking wants a click; interleaving them in one
handler is how a drag ends up selecting a region the user was scrolling
past. Shift is tracked as window state because a TouchArea's click carries
no modifiers.

Three states a layer can be in are worth distinguishing, and each has a
different remedy: stale needs re-segmenting, "no adjustment yet" needs a
slider moved, and the ordinary case needs nothing said. A bare selection
renders nothing and looks identical to a broken mask, which is the first
thing a new user will hit.

Known rough edge, commented where it happens: segmentation blocks the UI
thread for about half a second. Moving it to a worker needs the develop
session — GPU resources behind a RefCell shared with every callback — to
be reachable from another thread, which is a restructuring rather than a
change to the call. The button says "Finding regions…" first so the stall
is announced rather than looking like a hang.

ui/dr-ui/src/develop.rs

@@ -770,6 +770,35 @@ impl DevelopSession {
             .collect()
     }
 
+    /// What kind of mask a layer is — "regions", "linear", "radial".
+    pub fn mask_kind(&self, id: &str) -> &'static str {
+        self.graph
+            .masks()
+            .get(id)
+            .map_or("", |l| l.source.kind())
+    }
+
+    pub fn mask_inverted(&self, id: &str) -> bool {
+        self.graph.masks().get(id).is_some_and(|l| l.invert)
+    }
+
+    pub fn mask_opacity(&self, id: &str) -> f32 {
+        self.graph.masks().get(id).map_or(1.0, |l| l.opacity)
+    }
+
+    /// Whether a layer has any adjustment on it yet.
+    ///
+    /// Distinct from `is_active`, which also asks whether the layer is enabled
+    /// and visible. The panel wants specifically "you have made a selection
+    /// and not yet done anything with it", because that state looks identical
+    /// to a broken mask and is the most likely thing a first-time user hits.
+    pub fn mask_is_adjusted(&self, id: &str) -> bool {
+        self.graph
+            .masks()
+            .get(id)
+            .is_some_and(|l| l.active_ops().next().is_some())
+    }
+
     pub fn active_mask(&self) -> Option<&str> {
         self.active_mask.as_deref()
     }