G10: the theme's chrome fades through OKLab, easing in and out

ChromeTheme.interpolate mixes each colour in OKLab (colors.mixOklab), so a
fade between hues keeps its lightness instead of going dark and muddy mid
way; animation.Fade eases it in and out (smooth), and one retargeted mid
fade eases out from where it shows, so it never stalls. Still 10 frames
(160 ms). The page, text and syntax colours change at once as before.

Tests: OKLab round-trips its ends exactly and keeps a red-to-green mid-way
brighter than the sRGB mix. Gates as G9 (no golden moved). Feel review:
.scratch/render/crossfade/.

1 files changed, 10 insertions, 0 deletions

diff --git a/docs/effects.md b/docs/effects.md
index 61b969c2..0e0ad51a 100644
--- a/docs/effects.md
+++ b/docs/effects.md
@@ -43,6 +43,7 @@ why. The verdict is the user's.
 | G7 panel transitions | `PanelSlide`, `PanelZoom`, `PanelVertical`, `PanelDissolve` (off) with `Motion` | a moving pane casts a shadow; a retargeted move keeps its speed; the GUI's dissolve is a crossfade | .scratch/render/transitions/ (slide smooth, bouncy, retargeted; dissolve) | pending |
 | G8 notice chips | follows `Motion` and `Lift` | a notice floats in tier 4 on a lift (shadow) while Lift is on, drops in easing out and dissolves easing in, quicker than it came (150 ms against 180) | .scratch/render/notices/ (acme, forge) | pending |
 | G9 selection glow | `SelectionGlow on\|off` (off) | a soft halo of the selection's colour round each block of it in a body, fading in over 100 ms | .scratch/render/selection/ (forge, acme, lapis; lapis-glow-zoom.png) | pending |
+| G10 theme crossfade | always (a theme change) | the chrome's colours fade through OKLab, easing in and out (out when retargeted) | .scratch/render/crossfade/ (forge, lapis, acme cut short by dusk) | pending |
 
 ## Motion flavours
 
@@ -197,3 +198,12 @@ below min(its contrast, 4.5); it comes up over 100 ms with an ease-out
 panes move. The blocks are read from the cells painted in the theme's
 selection colour, a pixel shell's inference (§1.5, smell 4) until
 selections are regions.
+
+Notes on G10: a theme change already faded the chrome (tags, rules, grips,
+rails) over 10 frames (160 ms, in §8.1's 150 to 250 for a state change),
+mixing sRGB bytes linearly in time. Now each colour goes through OKLab, so
+a fade between two hues keeps its lightness instead of dipping dark and
+muddy, on a smooth ease-in-out; a change made mid-fade sets off from where
+it shows and eases out, so it moves at once instead of stalling. The page,
+its text and the syntax colours still change at once with the theme, as
+before: fading them would re-resolve every cell each frame.