wallpaper: one monitor to preview, a line per transition

Selecting was unclear with a tile doing both jobs. A mock monitor now
plays whatever line is clicked, and a click changes nothing else; the
choice is an Active mark per line, and Random's pool a Switch beside it.
Each line shows a frame caught halfway through its transition.

MockScreen takes children, drawn on its panel.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

1 files changed, 4 insertions, 2 deletions

diff --git a/desktop/modules/appearance/README.md b/desktop/modules/appearance/README.md
index ab61811..2c9a6ad 100644
--- a/desktop/modules/appearance/README.md
+++ b/desktop/modules/appearance/README.md
@@ -162,8 +162,10 @@ Apply, and every engine change, plays a transition over the swap, the old
 picture giving way to the new one (`WallpaperSpread.qml`, `WallShader.qml`,
 one `.frag` each: burn, ink, shatter, ripple, melt, blinds, crosswarp). The
 Transitions page picks one, or Random, which draws from the ticked ones and
-never repeats the last. Clicking a tile plays it between the two current
-wallpapers through the same shader. Both screens play the same one in a run.
+never repeats the last. Each line shows a frame halfway through its
+transition; clicking one plays it on the page's mock monitor, between the two
+current wallpapers, through the same shader, and only the Active mark changes
+the choice. Both screens play the same one in a run.
 Each shader must draw the old picture exactly at `progress` 0 and the new one
 at 1, or the cover pops as it comes and goes. Pictures reach a shader through
 a layer, since an `Image` sampled directly gives the whole picture and not its