Let the curve widget plot whichever curve is asked for

An operation with four curves and a panel that draws one plot needs a way to
say which. The panel finds out the way it finds out everything else: the
points are faceted with the subject they act on, consecutive parameters
sharing a subject are one curve, and a widget spanning several of them gets
a selector over their names. Nothing in ui/ contains the word "red", and an
operation that grows a fifth curve arrives with a fifth chip.

The names ride on the panel rather than on the curve's row, because a Slint
model is compared by identity: a fresh list built on every parameter event
would make the row look changed every time, and rewriting a row rebuilds the
element holding the drag in progress. That is the hazard the in-place point
update already exists to avoid.

Which curve is on show is interface state, not an edit. It changes no pixel,
so it takes no history step, reaches no sidecar, and redraws nothing — the
photograph on screen is already right.

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

ui/dr-ui/src/labels.rs

@@ -54,6 +54,19 @@ pub fn resolve(key: &str) -> String {
         "param.channel.sat" => "Saturation".into(),
         "param.channel.lum" => "Luminance".into(),
 
+        // The tone curve's four curves, which its points are *subject* to.
+        //
+        // Catalogued rather than derived because the master curve's key would
+        // otherwise read "Rgb": these are the terms of a four-way choice, and
+        // one of them miscapitalised is the one the eye goes to. The three
+        // colours would derive correctly and are written out beside it anyway,
+        // since a list where one entry is translated and three are guessed is
+        // the shape a half-finished translation takes.
+        "channel.rgb" => "RGB".into(),
+        "channel.red" => "Red".into(),
+        "channel.green" => "Green".into(),
+        "channel.blue" => "Blue".into(),
+
         // The hue bands, which a faceted row is *subject* to.
         //
         // Catalogued even where `derive` would produce the same word, because