Add Loop type (flywheel) + palette linter

- New type #27: Loop — ring of 5-8 stations around exactly one hub,
  solid flow arrows around the ring, dashed write-back spokes into
  the hub. Reference doc with ring layout math, budget, and
  anti-patterns; flagship example "The self-improving loop" in
  light/dark/full variants; gallery tab; README showcase under the
  hero image.
- scripts/lint-skin.py — contributor palette linter: reads allowed
  tokens from style-guide.md at runtime, flags off-palette hexes,
  pure black, external assets, scripts, and off-system fonts.
  Baseline file grandfathers pre-2.0 examples; --all --baseline is
  green.
- Linter immediately caught 36 near-miss hexes in 9 of the newly
  regenerated examples; swapped to exact series-palette tokens and
  recaptured the affected screenshots.

.codex-plugin/plugin.json

@@ -1,6 +1,6 @@
 {
   "name": "diagram-design",
-  "description": "Editorial-quality technical and product diagrams - 26 types rendered as standalone HTML with inline SVG, skinnable to match your brand",
+  "description": "Editorial-quality technical and product diagrams - 27 types rendered as standalone HTML with inline SVG, skinnable to match your brand",
   "version": "2.0.0",
   "author": {
     "name": "Cathryn Lavery",