fix(dashboard): exempt the banner buffer from auto-dim

The butterfly banner is a transparent PNG. On this X11 build Emacs composites image alpha against one background color and caches the flat pixmap. So when auto-dim remaps a non-selected dashboard's background to near-black, the cached image keeps its old composite and the transparent edges show as a lighter rectangle.

I exempted the *dashboard* buffer from dimming through the fork's never-dim-buffer hook, so its background never shifts. Live alpha compositing would need a pgtk build, which is out because of its fractional-scaling input lag, and every theme-level workaround changes dimming for all buffers. Scoping the exemption to one short-lived buffer is the narrow fix. The trade is no focus cue when the dashboard is shown in a split.

I also dropped the :mask heuristic prop from the prior banner commit. The PNG already carries a real alpha channel, so heuristic masking was the wrong tool. Once the background is stable, the native alpha over the theme background reads clean on its own. I added Normal/Boundary/Error tests for the predicate.

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