feat(website): asciinema cast pipeline + landing quickstart demo

Settle the cast toolchain (STYLE.md #9) and build the demo pipeline end to
end. Driver: autocast, chosen by spike — its !Interactive feeds keys to the
running TUI and captures the redraw, the right model for a full-screen
crossterm app. asciinema-automation was rejected (assumes shell echo/\n Enter;
produced a garbled cast against the TUI).

- add asciinema-player; Cast.astro (player island) + Demo.astro (the WASM-swap
  seam, ADR-website-001 §3)
- casts-src/: human-readable command-lists (casts.mjs) + generate.mjs, exposed
  as `pnpm casts`; expands steps to autocast YAML and records to public/casts/.
  Command-lists are the durable source; .cast files are regenerable (final
  re-record sweep due once the app is locked).
- quickstart.cast (create -> add columns -> insert -> show data) embedded on
  the landing page above the feature cards.

Verified: pnpm build clean (25 pages); player + cast bundled and served;
landing HTML references the cast. Visual playback check pending (no headless
browser here — verify via dev server over the tunnel).

website/package.json

@@ -8,12 +8,14 @@
     "start": "astro dev",
     "build": "astro build",
     "preview": "astro preview",
+    "casts": "node casts-src/generate.mjs",
     "astro": "astro"
   },
   "dependencies": {
     "@astrojs/starlight": "^0.39.3",
     "@astrojs/starlight-tailwind": "^5.0.0",
     "@tailwindcss/vite": "^4.3.0",
+    "asciinema-player": "^3.15.1",
     "astro": "^6.3.1",
     "sharp": "^0.34.5",
     "tailwindcss": "^4.3.0"