Fade the shimmer's glyphs instead of tinting them

The shimmer did not sweep: every glyph drew at full strength and the line only twitched as it stepped. The shimmer dimmed glyphs by mixing in a same-colour highlight at lower alpha, which the carried gpui version applies without lowering the text’s own alpha, so nothing changed.

Each glyph now sets fade_out, which scales the alpha directly. The curve is the same: full strength on the crest, easing to half strength five characters away. Measured on the installed build, a bright crest moves across the dimmed text about every 1.8 s. A unit test runs the fade through gpui’s own highlight code so a later gpui change cannot silently undo it.