Show the colour mixer as three runs of twelve, each row a colour

The mixer was thirty-six sliders reading "Hue / Sat / Lum" twelve times
over with nothing saying which band any row belonged to. The identity was
there all along — the descriptor declares param.mixer.orange.sat and BANDS
carries orange at 30° — and was discarded on the way out: labels.rs had no
mixer entries, so every key fell through to a derived label that yields the
bare channel name.

A parameter can now say which aspect it adjusts and which subject it
adjusts it on, with the subject's hue where the subject is a colour
(descriptor::Facet). That is data about what the operation does, not a
layout: the mixer genuinely weights pixels around 30°. What to draw from
30°, and in what order to stack the runs, stay in dr-ui (ARCH §4.3a) —
develop.rs brings rows sharing an aspect together and marks the first of
each, and adjust.slint names the run once and draws a swatch, a track and a
readout on one line.

Grouped by channel rather than by band because an edit is almost never
"everything about orange"; it is the saturation of the greens, made by
comparing one channel across neighbouring bands. Twelve band sections put
those twelve rows in twelve different places.

The swatch is the label, which is what makes twelve rows fit where four
did. The band name is not lost: it is the row's accessible label, so the
control is not colour-only, and labels.rs is where the mapping is written
down — including chartreuse as "Yellow-Green" and spring as "Blue-Green",
since nobody hunting foliage scans a list for "Spring".

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

ui/dr-ui/ui/widgets.slint

@@ -455,6 +455,45 @@ export component Value inherits Text {
     vertical-alignment: center;
 }
 
+// The colour a row edits, as a small square: the label for a control whose
+// subject is a hue rather than a word.
+//
+// **The one place hue enters the chrome, and it is not an exception to the
+// palette rule so much as outside it.** The rule forbids colour used as
+// decoration — an accent on a heading, a tinted border — because a saturated
+// patch beside the photograph shifts how the photograph reads. This is the
+// same kind of thing as the image itself: data. A row that edits the 30° band
+// has to say *which* band, and no achromatic treatment can say "orange".
+//
+// **The core supplies degrees; everything else is decided here.** The
+// operation knows its band is centred at 30° because that is the number it
+// weights pixels around (ARCH §4.3a). Saturation and brightness are this
+// file's to choose, and they are chosen well short of full: a fully saturated
+// row of twelve squares is a paintbox sitting next to a print. Held back far
+// enough to read as instrument markings, and no further, since a swatch that
+// cannot be told from its neighbour has stopped identifying anything.
+export component Swatch inherits Rectangle {
+    /// Where the subject sits on the hue wheel, in degrees.
+    in property <float> hue;
+    /// Muted from a full-strength hue, so twelve of these read as a scale
+    /// rather than as a palette.
+    in property <float> saturation: 0.55;
+    /// Bright enough to separate from the surface it sits on, which is dark;
+    /// a swatch at full value would be the brightest thing in the panel and
+    /// outrank the modified marker.
+    in property <float> brightness: 0.85;
+
+    width: Theme.swatch;
+    height: Theme.swatch;
+    horizontal-stretch: 0;
+    border-radius: Theme.radius-sm;
+    background: hsv(root.hue, root.saturation, root.brightness);
+    // A hairline, so a dark swatch (a deep blue at this brightness) still has
+    // an edge against the surface and reads as a square rather than a smudge.
+    border-width: 1px;
+    border-color: Theme.rule;
+}
+
 // Supporting text: a hint under a field, a count beside a title, an empty
 // state's second line. The faintest ink, because it is there for the reader
 // who stopped to look and should not catch the eye of the one who did not.