feat: ✨ lazily-loaded Gerber viewer demo in the blog, served from R2

Embed KiCad's gerbview WASM in the graphics-to-WebGL blog post as a
click-to-load demo that opens the tiny_tapeout board entirely in-browser.

- GerberDemo.astro: lazy iframe embed + cross-origin-isolation reload guard
  + WebGL2/SharedArrayBuffer feature-detect with a poster fallback
- public/gerber-demo/{index.html,boot.js}: self-contained gerbview boot
  harness (config seed, MEMFS board preload, argv auto-open)
- board/ tiny_tapeout layers + poster.png committed; wasm/ glue JS committed
  (served same-origin — pthread worker can't be cross-origin), while
  gerbview.wasm + kicad-resources.bin are git-ignored and served from
  Cloudflare R2 (assets.pcbjam.com)
- COOP/COEP require-corp scoped to the post + /gerber-demo routes
  (dev: astro.config.mjs + middleware.ts; prod: vercel.json)
- post converted to MDX (@astrojs/mdx) so it can embed the component
- scripts/: sync-demo-wasm.sh, r2-deploy.sh, r2-cors.json

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

site/package.json

@@ -14,6 +14,7 @@
     "astro": "astro"
   },
   "dependencies": {
+    "@astrojs/mdx": "^6.0.3",
     "@astrojs/vercel": "^10.0.8",
     "@fontsource/oswald": "^5.2.8",
     "@vercel/analytics": "^2.0.1",