Generative: the embedding turns by drag past its edge; version diffs are tested

The turned embedding turns through gpui's drag, marked with its view, so a
drag keeps turning past the plot's edge. What a chosen version is set
against, and its heading, move into versions::compared, with a test. A
hidden legend entry dims its key and not its ring, and an embedding refuses
a group named twice.

src/generative/lens.rs

@@ -11,6 +11,8 @@ const EYE: f32 = 4.0;
 pub(crate) const START: (f32, f32) = (0.6, 0.35);
 /// What a key turns the view.
 pub(crate) const STEP: f32 = PI / 12.0;
+/// What a drag turns the view for each pixel.
+pub(crate) const DRAG: f32 = 0.01;
 /// Farthest the view tips up or down, short of looking straight along the upright axis.
 const TIP: f32 = 1.4;