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/src/content/docs/index.mdx

@@ -14,6 +14,7 @@ hero:
 ---
 
 import { Card, CardGrid, LinkCard } from '@astrojs/starlight/components';
+import Demo from '../../components/Demo.astro';
 
 RDBMS Playground is a cross-platform terminal app that gives you a safe
 sandbox for exploring relational database concepts: tables, columns,
@@ -21,6 +22,8 @@ primary and foreign keys, relationships, indexes, queries, and query
 plans. It is built for learning — from your first table to writing raw
 SQL.
 
+<Demo src="/casts/quickstart.cast" title="From a fresh table to your first query, in simple mode." />
+
 <CardGrid>
 	<Card title="Two ways to work" icon="seti:db">
 		Start in **simple mode** — a friendly, keyword-based command language —