Mask the subject the model found, not the regions underneath it

The watershed hierarchy does not survive a photograph, so local masking
stops depending on it. A layer can now be one recognised object, and the
object's own coverage is the mask.

`Options::watershed` defaults off. It costs ~80 ms plus a full-resolution
readback to produce a ladder that collapses, and paying that on every
photograph buys a control that misleads. Kept switchable rather than
deleted: the passes and the hierarchy are correct in themselves and it is
the merge criterion that fails, which is a change to one function.

Masks now rasterise in **source** space at proxy resolution and are sampled
by the composed shader after the framing map. That fixes a real bug: they
were rasterised in output space, so zooming slid the photograph underneath
a mask that stayed pinned to the viewport, and cropping moved every
adjustment to a different part of the picture. Doing it this way also
leaves the framing map in exactly one place — a second copy in the mask
shader would have been a second thing to keep in step, failing only when
straightened.

A subject is stored as identity, not pixels: the mask is megabytes and is
reproducible by running the same model over the same image, so the sidecar
carries the index, the class and the score, and the session carries the
pixels. The class is there to be checked — if instance 3 comes back a "car"
where it was a "dog", something changed and the layer is stale rather than
silently masking the wrong thing.

The overlay now draws instances and is transparent everywhere else. The
region version covered every pixel and so hid the photograph it was drawn
over; the question it exists to answer is whether an outline follows the
subject, which you can only answer by seeing both.

`examples/local.rs` is the worked example: subject in colour with the rest
monochrome, and the subject lifted out of its background. Run on a 5472x3648
CR2 it finds two people and two cars, and the colour-pop keeps her hat and
hair while the wall and grass behind go grey.

ui/dr-ui/src/masks_ui.rs

@@ -31,6 +31,7 @@ pub(crate) fn sync(window: &AppWindow, session: &Rc<RefCell<Option<DevelopSessio
         window.set_subject_rows(ModelRc::new(VecModel::<SubjectRow>::default()));
         window.set_segmented(false);
         window.set_segmentation_regions(0);
+        window.set_has_regions(false);
         window.set_editing_mask(false);
         window.set_overlay_on(false);
         return;
@@ -69,6 +70,7 @@ pub(crate) fn sync(window: &AppWindow, session: &Rc<RefCell<Option<DevelopSessio
     window.set_segmented(s.has_segmentation());
     window.set_segmentation_level(s.segmentation_level() as i32);
     window.set_segmentation_regions(s.segmentation_region_count() as i32);
+    window.set_has_regions(s.has_regions());
     window.set_editing_mask(active.is_some());
 
     // The overlay is regenerated only when there is one to draw. It is a
@@ -322,6 +324,7 @@ pub(crate) fn reset(window: &AppWindow) {
     window.set_overlay_on(false);
     window.set_segmenting(false);
     window.set_segmented(false);
+    window.set_has_regions(false);
     window.set_mask_rows(ModelRc::new(VecModel::<MaskRow>::default()));
     window.set_subject_rows(ModelRc::new(VecModel::<SubjectRow>::default()));
     window.set_editing_mask(false);