Add depth and light: glow, tiles, avatars, stats, steps, decks, skeletons, reveal

Tokens
- --nv-color-glow role (light and dark) and a glow scale: --nv-glow-sm/md/lg,
  --nv-glow-blur-*, --nv-glow-spread-*, --nv-glow-strength
- --nv-stagger (0ms under reduced motion) and --nv-duration-shimmer

Components
- Card: --lit (lit top edge in --nv-card-tint), --glow, --spotlight
  (light follows the pointer); light layers combine with every elevation
- Tile: small raised key for a glyph or product mark; --solid, --glow, sizes
- Avatar: raised disc with --ring / --glow, sizes, __status, avatar group
- Stat: readings set into one recessed well (.nv-stat-group)
- Steps: markers set into a rail; done steps light their stretch; vertical
- Deck: one surface in front of stacked layers of its kind
- Skeleton: recessed placeholder with a slow sweep, still under reduced motion
- Text: .nv-text-lit and .nv-text-lit--accent

Continuity
- .nv-reveal / .nv-reveal-group: content arrives once as it scrolls into view
  (nv-arrive keyframe); niveus.js marks pending state, so nothing is hidden
  without the script or under reduced motion
- niveus.js tracks the pointer for .nv-card--spotlight

README and REFERENCE document every new class, token and attribute;
tools/check.py passes and dist/ is rebuilt.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

niveus/components/deck.css

@@ -0,0 +1,55 @@
+/* ==========================================================================
+   Niveus — Deck
+   --------------------------------------------------------------------------
+     <div class="nv-deck">
+       <article class="nv-card">…</article>
+       <div class="nv-deck__layer" aria-hidden="true"></div>
+       <div class="nv-deck__layer" aria-hidden="true"></div>
+     </div>
+
+   One surface in front of others of its kind: the current item of a set that
+   has more behind it. The layers are cards seen edge-on, stepping back and
+   dimming with distance, so the set has depth without anything having to
+   move. Use it where "there are more of these" is part of the message — a
+   showcase of products, a stack of notifications, a carousel at rest.
+
+   Everything in the deck shares one grid cell. The front surface is any
+   element; the layers are decorative and hidden from assistive technology.
+   Panels that swap (an .nv-tab-panel set) can live in the deck together:
+   the hidden ones take no space.
+   ========================================================================== */
+
+.nv-deck {
+  --_step: var(--nv-space-3);
+
+  display: grid;
+  position: relative;
+  isolation: isolate;
+  /* Room above the front surface for the layers that peek out behind it. */
+  padding-block-start: calc(var(--_step) * 2);
+}
+
+.nv-deck > * {
+  grid-area: 1 / 1;
+  min-inline-size: 0;
+}
+
+.nv-deck__layer {
+  --_depth: 1;
+
+  z-index: -1;
+  pointer-events: none;
+  background-color: color-mix(in oklab, var(--nv-color-surface) 90%, transparent);
+  background-image: var(--nv-sheen-surface);
+  border: var(--nv-border-width) solid var(--nv-color-border-subtle);
+  border-radius: var(--nv-radius-lg);
+  box-shadow: var(--nv-bevel), var(--nv-outline), var(--nv-elevation-1);
+  translate: 0 calc(var(--_step) * var(--_depth) * -1);
+  scale: calc(1 - var(--_depth) * 0.05) 1;
+  opacity: calc(1 - var(--_depth) * 0.3);
+}
+
+.nv-deck__layer + .nv-deck__layer {
+  --_depth: 2;
+  z-index: -2;
+}