feat(theme-studio): auto-dim split preview

auto-dim-other-buffers is a package face, not a theme face, so build-inventory.el (it scans only elpa/straight packages) never listed it and the studio couldn't theme it. This adds it as a bespoke app.

The preview is a vertical split: the focused window on the left in real syntax colors, the same code on the right collapsed to the single auto-dim-other-buffers face, the way Emacs renders a non-selected window. Both panes follow the language selector. A trailing row shows auto-dim-other-buffers-hide, whose foreground matches the background so it vanishes when dimmed.

A #autodimtest gate covers the split, the uniform recolor, and language sync.

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