Reference and build, side by side development only, not shipped

Left: the reference demo. Right: the built standalone page. The list below is docs/reference-differences.md, to check off as you compare. Ticking a box here is not the sign-off; that is recorded in the document's last section.

Show both in

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Reference embedding-tumbler-demo.html

Build embeddings_tumbler/static/index.html

Intended differences

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