Function build_embedding_index 

pub fn build_embedding_index(
    provider: &dyn EmbeddingProvider,
    reader: &MicroscopeReader,
    max_depth: u8,
    output_path: &Path,
) -> Result<(), String>

Build a sparse embedding index file from a provider and reader. Only blocks at depth 0..=max_depth with non-trivial text get embedded; failed or zero embeddings are omitted (search treats them as absent).