wallpaper: play the transition by name

Transitions.next() is asked once per run and the name goes out through
swapping, so both screens play the same effect, and the cover runs it
through WallShader, which the settings page's thumbnails will share.
progress is a plain 0 to 1 for every shader now; burn.frag keeps the
range it was tuned for by remapping it.

newImg gains a layer so it can be a shader's target: an Image sampled
directly gives the whole picture, not its PreserveAspectCrop crop.

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

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