POLISH-3. Replaces the bare Inspector-mode row in the Words demo with a real
topbar: a document breadcrumb (FileText icon + the doc's first-heading title),
a "Saved / Editing…" indicator that flips on document change and settles after
a short idle, and the Inspector-mode + Edit/Preview toggles.
Preview mode renders the document as a clean, read-only reading view via the
export-only `serializeHtml` serializer (bare semantic HTML with its own
typography — headings, blockquote, code, lists, table, links). Showcases the
export path now that table cells serialize block content (<td><p>…</p></td>).
Also exposes the export-only serializers on the public soma barrel
(`serializeHtml`, `serializeMarkdown`, `renderWordsPlainText`) — apps need them
for preview / copy / download.
Browser-verified: topbar renders; Edit↔Preview swaps the editor for the
read-only article (table + headings render); Saved badge shows. npm run check:
0 new errors (1 pre-existing grafito baseline).
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>