Restyle on the Carbon Design System

Adopt Carbon at the design-token level (the component libraries ship JS and
target app UIs; this is a static content site):

- IBM Plex Sans/Mono, self-hosted via @fontsource (no external requests)
- Carbon color tokens + layered White / Gray 100 themes
- 8px spacing scale, productive/expressive type, motion tokens, square
  geometry, 2px focus ring
- UI-shell header, tile grid, Carbon tags, geometric severity meter,
  breadcrumb, colophon crediting Carbon
- Charts themed with IBM Plex + Carbon's data-visualization palette
  (validated colorblind-safe/contrast) on a fixed light canvas so the
  build-time SVG stays legible in both themes

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01XPEvmMbac2fCvKXpQcovj8

package.json

@@ -12,6 +12,8 @@
   },
   "dependencies": {
     "@astrojs/mdx": "^4.2.0",
+    "@fontsource/ibm-plex-mono": "^5.3.0",
+    "@fontsource/ibm-plex-sans": "^5.3.0",
     "astro": "^5.6.0",
     "vega": "^5.30.0",
     "vega-lite": "^5.21.0"