feat(website): relationship-diagram cast on the relationships page

Third earmarked cast: declare a 1:n relationship, then `show relationship`
draws the two-table connector diagram — showcases the V1 relationship
visualization with motion a still block can't. Convert the relationships
reference page to .mdx and embed it above the syntax (the static diagrams
below remain the exact reference).

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

website/casts-src/casts.mjs

@@ -17,6 +17,28 @@
 
 /** The shared library narrative, trimmed per cast. */
 export const casts = [
+  {
+    name: 'relationship-diagram',
+    title: 'Declare a relationship and see it drawn',
+    width: 90,
+    height: 26,
+    typeSpeed: '45ms',
+    steps: [
+      { wait: 900 },
+      { type: 'create table authors with pk author_id(serial)', after: 800 },
+      { type: 'add column to authors: name (text)', after: 900 },
+      { type: 'create table books with pk book_id(serial)', after: 800 },
+      { type: 'add column to books: title (text)', after: 700 },
+      { type: 'add column to books: author_id (int)', after: 1000 },
+      {
+        type: 'add 1:n relationship as books_author from authors.author_id to books.author_id on delete cascade',
+        after: 1400,
+      },
+      // the money shot: the two-table connector diagram
+      { type: 'show relationship books_author', after: 2600 },
+      { type: 'quit', after: 400 },
+    ],
+  },
   {
     name: 'assistive-editor',
     title: 'The input field helps as you type — completion and a live validity indicator',