feat(website): joins cast on the querying-with-joins guide

Fourth cast: build a minimal two-table schema with rows, switch to advanced
mode (`mode advanced`), and run a join pairing each book with its author —
shows the mode switch + SQL + multi-table result, motion that complements the
guide's static examples. Convert the guide to .mdx and embed above the intro.

Recorded via `pnpm casts`; build clean (25 pages).

website/casts-src/casts.mjs

@@ -17,6 +17,34 @@
 
 /** The shared library narrative, trimmed per cast. */
 export const casts = [
+  {
+    name: 'joins',
+    title: 'Switch to advanced mode and join across tables',
+    width: 90,
+    height: 26,
+    typeSpeed: '42ms',
+    steps: [
+      { wait: 900 },
+      // minimal two-table schema + a few rows to join
+      { type: 'create table authors with pk author_id(serial)', after: 650 },
+      { type: 'add column to authors: name (text)', after: 650 },
+      { type: 'create table books with pk book_id(serial)', after: 650 },
+      { type: 'add column to books: title (text)', after: 650 },
+      { type: 'add column to books: author_id (int)', after: 800 },
+      { type: "insert into authors (name) values ('Ursula K. Le Guin')", after: 600 },
+      { type: "insert into authors (name) values ('Italo Calvino')", after: 700 },
+      { type: "insert into books (title, author_id) values ('A Wizard of Earthsea', 1)", after: 600 },
+      { type: "insert into books (title, author_id) values ('The Left Hand of Darkness', 1)", after: 600 },
+      { type: "insert into books (title, author_id) values ('Invisible Cities', 2)", after: 900 },
+      // switch to SQL and join
+      { type: 'mode advanced', after: 1200 },
+      {
+        type: 'select authors.name, books.title from books join authors on books.author_id = authors.author_id order by authors.name',
+        after: 2600,
+      },
+      { type: 'quit', after: 400 },
+    ],
+  },
   {
     name: 'relationship-diagram',
     title: 'Declare a relationship and see it drawn',