Image Trace: the empty-palette test uses a library blue the row maps to

The test of a color whose every path straightens to no area traced a
pale blue row with a library of a light color, black and [40, 120, 240].
With Document Library weighing hue and chroma three times as much as
lightness, the row takes the light color instead of that blue, so the
test no longer traced any blue. The library blue is now [60, 130, 235],
which the row still maps to; the assertions are unchanged.

crates/trace/src/tests.rs

@@ -1458,7 +1458,7 @@ fn a_color_whose_every_path_straightens_to_no_area_is_left_out_of_the_palette()
     // A light rectangle with a black block on it and a one-pixel row of a pale blue along its lower
     // edge, on a transparent background. Traced with a library of the light, the black and a
     // blue, the row is the only part in the blue.
-    const BLUE: [u8; 3] = [40, 120, 240];
+    const BLUE: [u8; 3] = [60, 130, 235];
     let img = Raster::from_fn(200, 120, |x, y| {
         if (60..140).contains(&x) && (70..90).contains(&y) {
             BLACK