Give a mask an edge treatment, and find out the watershed has none worth having

Two things, and the second is why the first matters more than expected.

Mask layers gain a feather, a falloff curve and a morphology, all defined
against a signed distance from the boundary rather than as separate
features — one exact distance field answers "how soft" and "how far" at
once, so dilation is a threshold at -r, erosion one at +r, and closing and
opening are one of each in sequence. The compound pair costs a second
distance field, which is why they are named rather than presented as a
radius that happens to be signed. Types, defaults and sidecar round-trip
only; the field itself is next.

`edge-feather` and `edge-falloff`, not `feather` and `falloff`, because a
radial mask already writes `feather` for the fraction of its radius it
ramps over. Same word, different quantity, different units — sharing the
key would have made an existing file ambiguous.

The diagnostic that provoked this is committed as an ignored test, because
"does the ladder land on things a person means" is the question S15 exists
to answer and it should not depend on whoever still has the script. On
bus.jpg it answers badly: 35,075 regions at blur 2 over an 810x1080 frame,
and cutting that to 400 gives *one* region covering nearly the whole
picture plus 399 noise specks. Not over-segmentation — collapse. Almost
every saddle is near zero, so the merge order joins everything meaningful
before it joins anything spurious, and a global cut spends its entire
budget on grain.

So the granularity ladder does not currently work on a photograph, and the
region masks built on it inherit that. Recorded rather than worked around:
the next commits move local masking onto the model's instances, where the
edge treatment above is what makes a quarter-resolution mask usable.

ui/dr-ui/ui/masks.slint

@@ -213,7 +213,7 @@ export component MaskPanel inherits Rectangle {
         HorizontalLayout {
             PanelHeading { text: "LOCAL"; }
             Rectangle { horizontal-stretch: 1; }
-            if root.segmented: Value { text: root.region-count + " regions"; }
+            if root.segmented: Value { text: root.level + " / " + root.region-count; }
         }
 
         if !root.enabled: Caption { text: "No image"; }
@@ -264,19 +264,30 @@ export component MaskPanel inherits Rectangle {
         // Granularity, labelled by what it does rather than by its number:
         // "detail" is what a photographer is choosing between, where "300
         // regions" is an implementation detail they would have to learn.
+        // **The ceiling is the region count, not a constant.** It was 2000,
+        // and a photograph that segments into more than that had the finest
+        // part of its own ladder unreachable — the slider simply stopped
+        // before the regions did.
         if root.enabled && root.segmented: SliderRow {
             label: "Detail";
+            hint: "How finely a click divides the picture, out of "
+                  + root.region-count + " regions the watershed found.";
             value: root.level;
             default-value: 300;
             minimum: 8;
-            maximum: 2000;
+            maximum: max(root.region-count, 8);
             changed(v) => { root.level-changed(v); }
             reset => { root.level-changed(300); }
         }
 
         // --- what the model found ----------------------------------------
         if root.enabled && root.segmented && root.subjects.length > 0: Caption {
-            text: "Recognised";
+            // Says where these came from and how they differ from a region.
+            // A list of four beside a count of three thousand invites exactly
+            // one question, and the panel should answer it rather than
+            // provoke it.
+            text: "Subjects the model recognised. Click one to select the whole thing.";
+            wrap: word-wrap;
         }
 
         if root.enabled && root.segmented: VerticalLayout {