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/src/content/docs/using-the-playground/the-assistive-editor.mdx

@@ -5,9 +5,13 @@ sidebar:
   order: 2
 ---
 
+import Demo from '../../../components/Demo.astro';
+
 You type commands into the input field at the bottom of the screen, and it
 helps you as you go. None of this gets in your way — it is all advisory.
 
+<Demo src="/casts/assistive-editor.cast" title="Tab completes a table name; the [ERR] marker catches a misspelled one before you submit." />
+
 ## Completion
 
 Press <kbd>Tab</kbd> to complete what you are typing. Completion knows about