feat(writekit): a way out of atoms, and a slash menu that behaves

Two gaps an author hits within the first minute of using atom blocks:

- there was no way to add a paragraph after a non-text block. Enter on a
  selected atom now starts a paragraph below it (exitAtom, chained before
  splitBlock), and a click on the root's padding below a trailing atom
  does the same — ends-in-text just places the caret at the end
- the slash menu ignored its own overflow: keyboard navigation walked the
  highlight out of view (now scrollIntoView nearest), and a background
  wheel scroll tore the menu off its caret anchor (now prevented outside
  the menu; scrolling the list itself stays native)
- BlockMeta gains `description`; the menu shows a detail pane beside the
  list with the highlighted item's description, replaceable wholesale via
  the new #preview slot. Headless stays headless: the pane is unstyled
  text and appears only when there is a description or a slot. Preset
  blocks are all described

Both floating menus (slash, bubble) are re-layered to the combobox
convention: PopperRoot provides the positioning context OUTSIDE a bare
Portal, which resolves its target from the ConfigProvider's
teleportTarget — the previous hardcoded to="body" was overriding the
app's configured target. A Combobox itself is the wrong base here on
purpose: its keyboard lives on ComboboxInput, while a suggestion menu
must leave focus in the contenteditable — the editor is the input.

writekit 0.0.3.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

vue/writekit/src/blocks/heading.ts

@@ -14,5 +14,5 @@ export const heading = defineBlock({
     parseDOM: LEVELS.map(level => ({ tag: `h${level}`, attrs: { level } })),
   },
   inputRules: LEVELS.map(level => ({ match: new RegExp(`^#{${level}}\\s$`), attrs: { level } })),
-  meta: { title: 'Heading', icon: 'heading', keywords: ['heading', 'title', 'h1', 'h2', 'h3'], group: 'basic' },
+  meta: { title: 'Heading', icon: 'heading', keywords: ['heading', 'title', 'h1', 'h2', 'h3'], group: 'basic', description: 'A section title, levels 1–6.' },
 });