Put the refinement on a slider, per layer

The evidence was being gathered and spent immediately at one strictness
nobody could see or change. This makes it a control.

`MaskLayer::refine` is shaping, like the feather, and it lives on the layer
for the layer's reason: two layers may sit on the same category and want
different amounts of it, and the model ran once for both.

## What the segmentation now stores

`CategorySummary` keeps the **coarse** mask and the `Refinement` beside it,
rather than a refined mask. That is what gives the control an off position
that is bit-for-bit the model's own weighting, and what stops a strictness
change from needing the model again.

`category_mask_at` borrows at zero and wherever no refinement could be
fitted, so a layer nobody has touched costs nothing over the old path.

The fit moved to the far side of the orientation permutation. The verdict is
a per-pixel field over the same grid as the mask it gates, so fitting it
upright would mean permuting a proxy-sized buffer afterwards to match — a
second rotation, and a second chance to get one wrong. One logit cell is the
same number of pixels either way: the letterbox scales by the longer edge and
a permutation does not change which edge that is.

The two frames became a `Frames` struct rather than six parameters. This
function reads the picture twice for opposite purposes — the model needs it
upright or it recognises far less, the refinement needs the sensor's grid —
and a transposed pair produces a plausible mask over slightly the wrong
pixels, which is the failure this module is most prone to.

## It rebuilds the field, and the signature says so

`refine` is mixed into `subject_signature`. Unlike a feather, which is read
off a field that is already correct, this changes which pixels are in the
mask at all — so it changes the coverage the field is measured from. Omitting
it is the bug where the slider moves and nothing happens until some unrelated
control invalidates the cache.

That puts it in the same cost class as a close or an open, which is why the
row takes `SliderRow::changed` — already once-per-gesture, since that row
takes `SliderTrack`'s `committed` internally — rather than a live stream.

The slider is offered only where there is something to move: a category
source, *and* a refinement the frame actually gave enough to fit. A control
that moves and does nothing is worse than an absent one.

## Two defaults that are deliberately different

A layer added from the panel starts at 4.0, because a category's edges are
twenty proxy pixels wide before anything is done to them and a photographer
adding a sky mask wants the sky rather than the sky plus every chimney in it.

A layer read from a sidecar with no `refine` key starts at **zero**. A file
written before this control existed has to render as it did then, and a
default of 4 on absence would quietly re-grade every stored category mask in
the catalogue. `a_categorys_refine_strictness_survives_and_defaults_off`
holds both halves, and `an_out_of_range_refine_is_clamped` holds the file to
the scale — past the top of it every colour fails and the mask deletes
itself, which reads as lost work rather than as a bad file.

`MAX_REFINE` is dr-pipeline's own constant mirroring
`dr_segment::STRICTNESS_MAX`, following `Falloff` and `Morphology`: this
crate holds the description of an edit and must not depend on the crate that
runs a model. dr-ui is where the two meet, and the only place that converts.

Verified: fmt clean, clippy --workspace -D warnings clean, 488 dr-pipeline
and 60 dr-segment tests.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

ui/dr-ui/ui/masks.slint

@@ -29,6 +29,12 @@ export struct MaskRow {
     /// Index into `Morphology::ALL`.
     morphology: int,
     morph-radius: float,
+    /// How strictly a category is cut back to the pixels whose colour agrees
+    /// with it, in nats. Zero is the model's own weighting.
+    refine: float,
+    /// Whether this layer has a colour model fitted to refine against. Only a
+    /// category has one, and only where the frame gave enough to fit it.
+    refinable: bool,
     /// Whether the edge controls apply. A gradient has its own geometry and
     /// does not go through the distance field, so showing them would offer
     /// controls that do nothing.
@@ -97,6 +103,7 @@ component MaskEntry inherits Rectangle {
     callback invert-toggled(bool);
     callback opacity-changed(float);
     callback feather-changed(float);
+    callback refine-changed(float);
     callback falloff-picked(int);
     callback morphology-picked(int);
     callback morph-radius-changed(float);
@@ -235,6 +242,29 @@ component MaskEntry inherits Rectangle {
                 clicked => { root.refined(); }
             }
 
+            // Which pixels are in the mask at all. Above the edge controls
+            // because it decides the outline they then treat, and unlike them
+            // it rebuilds the distance field — the same cost as a close or an
+            // open, and the reason it commits on release rather than on every
+            // sample of a drag.
+            if root.data.refinable: SliderRow {
+                label: "Refine";
+                hint: "Drop pixels whose colour does not match the rest of "
+                      + "the category — a flag in the sky, a chimney, a bare "
+                      + "branch. Zero is the model's own outline.";
+                value: root.data.refine;
+                default-value: 4;
+                minimum: 0;
+                maximum: 8;
+                // Tenths, not whole nats. The interval between a flag going
+                // and a cloud going is under three nats wide, so whole steps
+                // would give the control about three useful positions inside
+                // the range it exists to explore.
+                precision: 1;
+                changed(v) => { root.refine-changed(v); }
+                reset => { root.refine-changed(4); }
+            }
+
             // The edge. All of it reads one signed distance field, so these
             // are free to move — none of them recomputes anything except a
             // compound morphology.
@@ -309,6 +339,7 @@ export component MaskPanel inherits Rectangle {
     callback mask-invert-toggled(string, bool);
     callback mask-opacity-changed(string, float);
     callback mask-feather-changed(string, float);
+    callback mask-refine-changed(string, float);
     callback mask-falloff-picked(string, int);
     callback mask-morphology-picked(string, int);
     callback mask-morph-radius-changed(string, float);
@@ -561,6 +592,7 @@ export component MaskPanel inherits Rectangle {
             invert-toggled(on) => { root.mask-invert-toggled(mask.id, on); }
             opacity-changed(v) => { root.mask-opacity-changed(mask.id, v); }
             feather-changed(v) => { root.mask-feather-changed(mask.id, v); }
+            refine-changed(v) => { root.mask-refine-changed(mask.id, v); }
             falloff-picked(i) => { root.mask-falloff-picked(mask.id, i); }
             morphology-picked(i) => { root.mask-morphology-picked(mask.id, i); }
             morph-radius-changed(v) => { root.mask-morph-radius-changed(mask.id, v); }