feat(theme-studio): flag unused palette tiles and columns

I added usedPaletteHexes, a reverse lookup over the syntax, ui, and package assignments (plus the ground endpoints) that resolves each reference to a hex. renderPalette outlines a tile whose color is referenced nowhere and outlines a whole column when none of its colors are used, so dead colors stand out for pruning before a theme ships. The check is biased safe: an unresolvable reference marks nothing, so a color that is actually used is never flagged.

Node tests cover the lookup. A #unusedtest gate covers the tile and column flags.

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