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/tokens/color.css

@@ -227,6 +227,11 @@
   --nv-color-sheen-strong-top: light-dark(var(--nv-light-sheen-strong-top), var(--nv-dark-sheen-strong-top));
   --nv-color-sheen-strong-bottom: light-dark(var(--nv-light-sheen-strong-bottom), var(--nv-dark-sheen-strong-bottom));
 
+  /* Glow: light a surface gives off, as opposed to the shade it casts. It is
+     the accent seen through air — strong enough in dark to read as a lit
+     edge, faint in light, where paper does not emit. */
+  --nv-color-glow: light-dark(var(--nv-light-glow), var(--nv-dark-glow));
+
   /* A knob: the thumb of a switch or a slider. It stays light in both
      appearances, the way a physical knob does not change color with the room. */
   --nv-color-knob: light-dark(var(--nv-light-knob), var(--nv-dark-knob));