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/casts-src/casts.mjs

@@ -0,0 +1,39 @@
+/**
+ * Cast source definitions — the durable, human-readable source for the
+ * asciinema demos (ADR-website-001 §2). `pnpm casts` runs `generate.mjs`,
+ * which turns each definition into an autocast YAML and records it to
+ * `public/casts/<name>.cast`. Edit these and re-run to re-record as the app
+ * evolves; the `.cast` files are regenerable artifacts.
+ *
+ * Step kinds:
+ *   { wait: ms }                      — pause (shown in the recording)
+ *   { type: 'text', after: ms }       — type the text + press Enter, then pause
+ *   { type: 'text', enter: false }    — type without pressing Enter
+ *   { key: 'Tab'|'Enter', after: ms } — press a single named key
+ *
+ * Every cast must end by quitting the app (a `quit` step) so the driver
+ * returns to the shell prompt.
+ */
+
+/** The shared library narrative, trimmed per cast. */
+export const casts = [
+  {
+    name: 'quickstart',
+    title: 'RDBMS Playground — first table to first query',
+    width: 90,
+    height: 26,
+    typeSpeed: '45ms',
+    steps: [
+      { wait: 1100 },
+      { type: 'create table authors with pk', after: 1000 },
+      { type: 'add column to authors: name (text)', after: 850 },
+      { type: 'add column to authors: birth_year (int)', after: 900 },
+      {
+        type: "insert into authors (name, birth_year) values ('Ursula K. Le Guin', 1929)",
+        after: 1300,
+      },
+      { type: 'show data authors', after: 1800 },
+      { type: 'quit', after: 500 },
+    ],
+  },
+];