Function digest_once 

pub fn digest_once(
    text: &str,
    origin: Option<Locator>,
    breaker: &mut Breaker,
    cached: &mut dyn FnMut(&B3Hash) -> Result<Option<Digest>, AxError>,
    write_prose: &mut dyn FnMut(&str) -> Result<String, AxError>,
) -> Result<DigestOutcome, AxError>

Digests one document, once in its life.

cached answers what the store already holds for this content hash; write_prose is the model call, which the caller owns. The order is the whole policy: hash, ask the cache, read the structure, and only then spend a model call — and skip that call entirely once the breaker is open.

Errors

Propagates a cache read that fails. A prose failure is not an error here: it becomes DigestOutcome::Structural, because a document whose summary failed is still a document with headings.