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/lib.rs

@@ -32,6 +32,7 @@ mod library_ui;
 mod live_style;
 mod net_runtime;
 mod presets;
+mod masks_ui;
 mod segmentation;
 mod settings_store;
 mod settings_ui;
@@ -282,6 +283,11 @@ fn reset_view_state(window: &AppWindow) {
     // beside the next one's filename is a confident, precise lie, and the gap
     // before the new frame settles is exactly long enough to read it.
     window.set_histogram(histogram::empty());
+    // TRACES: FR-DEV-3
+    // The region map belongs to one photograph. Carrying the stack, the
+    // overlay or the crosshair to the next one would offer a selection of
+    // regions that are not in the picture on screen.
+    masks_ui::reset(window);
 }
 
 /// Push the framing back to the geometry panel.
@@ -1723,6 +1729,12 @@ pub fn run(paths: Vec<PathBuf>) -> Result<()> {
             redraw(&w);
         });
     }
+    // TRACES: FR-DEV-3
+    // The local-adjustment panel. Wired as a block rather than inline because
+    // it is a dozen callbacks that all say the same three things, and they
+    // read better beside each other than scattered through this function.
+    masks_ui::wire(&window, &session, &rows, &redraw, gpu.clone());
+
     {
         let weak = window.as_weak();
         let session = session.clone();