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/src/content/blog/porting-kicad-graphics-to-webgl-with-claude.md

@@ -1,7 +0,0 @@
----
-title: "Porting KiCad's Graphics to WebGL with Claude"
-description: "Coming soon."
-pubDate: 2026-06-07
----
-
-Coming soon.

site/src/content/blog/porting-kicad-graphics-to-webgl-with-claude.mdx

@@ -0,0 +1,22 @@
+---
+title: "Porting KiCad's Graphics to WebGL with Claude"
+description: "Coming soon."
+pubDate: 2026-06-07
+---
+
+import GerberDemo from '../../components/GerberDemo.astro';
+
+{/* TODO(author): intro — the gerbview port and the graphics-abstraction-layer → WebGL work. */}
+
+Coming soon.
+
+## See it run: a Gerber viewer in your browser
+
+The viewer below is KiCad's `gerbview` compiled to WebAssembly. Click to load it
+(it streams ~22 MB on demand) and it renders a real multi-layer board —
+the [tiny_tapeout](https://tinytapeout.com/) demo — entirely client-side, with no
+server doing the rasterization.
+
+<GerberDemo />
+
+{/* TODO(author): the rest of the article. */}