feat(website): assistive-editor cast content (completes c904dbb)

The previous commit captured only the .md→.mdx rename — a botched `git add`
(a stale .md pathspec aborted the whole add) dropped the actual content. This
adds it:

- casts.mjs: the assistive-editor cast definition (Tab completion → the [ERR]
  validity indicator catching a misspelled table → friendly error → corrected
  command). Behavior verified by a throwaway spike before scripting.
- public/casts/assistive-editor.cast (generated via `pnpm casts`)
- embed the cast under the intro on the assistive-editor page

Verified: pnpm build clean (25 pages); cast bundled, served, and referenced.
Visual playback check pending (verify via dev server/tunnel).

website/casts-src/casts.mjs

@@ -17,6 +17,28 @@
 
 /** The shared library narrative, trimmed per cast. */
 export const casts = [
+  {
+    name: 'assistive-editor',
+    title: 'The input field helps as you type — completion and a live validity indicator',
+    width: 90,
+    height: 26,
+    typeSpeed: '55ms',
+    steps: [
+      { wait: 1100 },
+      { type: 'create table books with pk', after: 850 },
+      { type: 'add column to books: title (text)', after: 1100 },
+      // Completion: type a partial table name, press Tab to complete it.
+      { type: 'show data bo', enter: false, after: 1100 },
+      { key: 'Tab', after: 1400 },
+      { key: 'Enter', after: 1600 },
+      // Validity indicator: a misspelled table flags [ERR] before you submit.
+      { type: 'show data boook', enter: false, after: 1900 },
+      { key: 'Enter', after: 1700 },
+      // The corrected command runs cleanly.
+      { type: 'show data books', after: 1600 },
+      { type: 'quit', after: 400 },
+    ],
+  },
   {
     name: 'quickstart',
     title: 'RDBMS Playground — first table to first query',