Offer the categories in the masks panel

Everything before this was reachable only from an example that writes PPMs.
This is the part a photographer can touch: a list under the subjects, click
one, get a mask layer for every pixel of that category.

## Under the subjects, and the order is the argument

Clicking the photograph is how a local adjustment usually starts, so the
things the model *found* come first. A category is the move you reach for
deliberately — grade the sky, not this one bird — and putting it second says
so without a word of explanation.

Coverage is shown for the same reason a subject's score is: it tells the
photographer whether a category is worth a click before they spend one
finding out.

## Not a new tab, which is what was asked for

A develop tab is derived from `Attribute`, not declared — the tabs exist
because operations claim an attribute, and no amount of Slint adds one. A
seventh attribute would have meant duplicating every adjustment once per
category, and the combinatorics get silly by the third.

Reached through masks instead, a category composes with every adjustment
that already exists, and inherits feather and falloff rather than needing
its own. What was described as "per-category sliders with feather and decay"
is exactly what this is; only the door is different.

## Verified as far as it can be here

Compiles, populates, round-trips, 560 dr-ui tests green. **Not clicked** —
synthetic input is blocked on this setup, so how it looks and feels is
unverified and wants a human at the window.

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

ui/dr-ui/src/masks_ui.rs

@@ -30,7 +30,7 @@ use slint::{ComponentHandle as _, Model as _, ModelRc, VecModel};
 
 use crate::develop::{Abandon, DevelopSession, RefinedInstance, Segmented, SessionId};
 use crate::segmentation;
-use crate::{sync_rows, AppWindow, GradientHandle, MaskRow, ParamRow, SubjectRow};
+use crate::{sync_rows, AppWindow, CategoryRow, GradientHandle, MaskRow, ParamRow, SubjectRow};
 
 /// What the adjust panel's heading says when the controls are global.
 ///
@@ -146,12 +146,28 @@ impl Running {
     }
 }
 
+/// A descriptor name as a list label.
+///
+/// Only the first letter, because the names in `models/scene/categories.txt`
+/// are already the words a photographer would use — "sky", "vegetation" — and
+/// a lookup table would be a second place to edit every time one is added.
+/// Title case on a multi-word name would be wrong anyway: "Swimming pool", not
+/// "Swimming Pool".
+fn category_label(name: &str) -> String {
+    let mut chars = name.chars();
+    match chars.next() {
+        Some(first) => first.to_uppercase().collect::<String>() + chars.as_str(),
+        None => String::new(),
+    }
+}
+
 /// Push every mask-related property from the session into the window.
 pub(crate) fn sync(window: &AppWindow, session: &Rc<RefCell<Option<DevelopSession>>>) {
     let slot = session.borrow();
     let Some(s) = slot.as_ref() else {
         window.set_mask_rows(ModelRc::new(VecModel::<MaskRow>::default()));
         window.set_subject_rows(ModelRc::new(VecModel::<SubjectRow>::default()));
+        window.set_category_rows(ModelRc::new(VecModel::<CategoryRow>::default()));
         window.set_segmented(false);
         window.set_adjust_scope(GLOBAL_SCOPE.into());
         clear_handles(window);
@@ -193,6 +209,19 @@ pub(crate) fn sync(window: &AppWindow, session: &Rc<RefCell<Option<DevelopSessio
         .collect();
     window.set_subject_rows(ModelRc::new(VecModel::from(subjects)));
 
+    // Already largest-first from the precompute, and already filtered to the
+    // ones with enough coverage to be worth a control.
+    let categories: Vec<CategoryRow> = s
+        .categories()
+        .iter()
+        .map(|c| CategoryRow {
+            name: c.name.as_ref().into(),
+            label: category_label(&c.name).into(),
+            coverage: c.coverage,
+        })
+        .collect();
+    window.set_category_rows(ModelRc::new(VecModel::from(categories)));
+
     window.set_segmented(s.has_segmentation());
     window.set_adjust_scope(scope_label(s).into());
     sync_handles(window, s);
@@ -636,6 +665,22 @@ pub(crate) fn wire(
             redraw(&w);
         });
     }
+    {
+        let weak = window.as_weak();
+        let session = session.clone();
+        let redraw = redraw.clone();
+        let rows = rows.clone();
+        window.on_add_category_mask(move |name| {
+            let Some(w) = weak.upgrade() else { return };
+            if let Some(s) = session.borrow_mut().as_mut() {
+                s.add_category_mask(&name);
+            }
+            sync(&w, &session);
+            sync_rows(&w, &rows, &session);
+            redraw(&w);
+        });
+    }
+
     {
         let weak = window.as_weak();
         let session = session.clone();
@@ -796,6 +841,7 @@ pub(crate) fn reset(window: &AppWindow) {
     window.set_segmented(false);
     window.set_mask_rows(ModelRc::new(VecModel::<MaskRow>::default()));
     window.set_subject_rows(ModelRc::new(VecModel::<SubjectRow>::default()));
+    window.set_category_rows(ModelRc::new(VecModel::<CategoryRow>::default()));
     window.set_adjust_scope(GLOBAL_SCOPE.into());
     clear_handles(window);
 }