Add a shared, pure ProseBlocks helper that splits trusted plain text on
blank lines into <p> blocks and single newlines into <br>, with no markdown
library and no dangerouslySetInnerHTML. A catalog-whitelisted linkifier turns
grounded /learn/<slug> references into safe anchors via learn-more-link;
unknown slugs stay plain text. Rewrite StreamedProse to be streaming-safe:
completed paragraphs settle while only the growing tail animates word by
word.