vector index

It should be doable to add cosine similarity search. Making the document ingest incremental might be the challenging part.

For now, [this technique illustrated in the Fireproof alpha codebase](https://github.com/fireproof-storage/fireproof-alpha/blob/main/packages/fireproof/test/vector.test.js) is still relevant and will get you a working vector index as an add-on.

Start by reading the linked Fireproof alpha example in packages/fireproof/test/vector.test.js and compare its vector-index flow with this repository. Determine how incremental document ingest and cosine similarity should fit together; done means a working add-on vector index with cosine search and coverage for ingestion updates.

Written by the indexing model from the issue text.