feat(theme-studio): nerd-icons gallery as a hue-ordered icon grid

The nerd-icons pane is now a grid: one row per color face, the rows ordered by hue so families cluster, distinct icons (deduped within a color) drawn in their color with the icon's nerd-font name beneath. A "preview:" dropdown above the grid picks the glyph size in points, with Left/Right arrows to step it. Single-pane apps show it disabled, naming the preview. This replaces the v1 legend in the pane, whose data is still captured for round-trip.

build-nerd-icons-legend.el is now a library. A cj/nerd-icons-write-legend entry point requires nerd-icons only at write time, so the capture logic loads and unit-tests without it. It dedupes icons by name within a face, computes each face's native hue, and orders the groups by hue. Writing the test surfaced a latent bug: face-hsl used (cadr (assoc t spec)), which grabs the first keyword instead of the plist. It only worked because the real faces fall through to the face-foreground branch. I fixed it to a correct t-clause parse.

Coverage: 7 ERT capture tests (dedupe, hue order, lightness tiebreak, name sort, skip rules), 4 Python validator edges, and browser gates for the grid and the size dropdown.

Locate stays color-level: clicking a color flashes its icons, and clicking an icon flashes its color row. Icons aren't individually editable, so there's nothing per-icon to select.

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