feat(gallery): slide-rule unit stops, zoom-safe clicks, and four faces

Clicks compared e.clientX (visual px, scaled by the size toggle's CSS zoom) against layout-px mark coordinates, so at M or L size nearest-mark snapping picked the wrong stop, usually the far end — the dial felt impossible to select. The click handler now divides the zoom out (offsetWidth over rect width), and the widget shows a subtle outline on click focus so it reads as armed for arrow keys.

The printed numerals are now majors with minor ticks at the integer units between them, all selectable: click a numeral, a mark, or between marks, and arrows step one unit instead of one numeral. The spec sheet claimed drag-only input, which was never true; it now names the click and key idioms.

The face rides new --tn-* vars and GW.slideRule gains opts.skin plus setStyle backed by GW.slideRule.STYLES: warm backlit (shipped default), chrome with dark ink, 80s black glass with an LED-red needle, and marantz blue with a pale needle. Card 25 joins the styleChips rig.

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