Node data is embedded as JSON inside a <script> element, but the serializers
escaped only `</`. An unclosed `<!--` in one label followed by a later `<script`
drove the HTML tokenizer into script-data-double-escaped state, where the real
</script> no longer closed the tag and graph.html rendered blank. Escape every
`<` as < in both exporters/html.py and tree_html.py; labels still render
because `<` round-trips through JSON.parse.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
