Cached chunk embeddings change what a long document answers with: BM25 first,
embedding uncached chunks (or the whole document) in the background so the next
question on the same doc scores hybrid. selectText now returns its mode and the
number of chunks that had a vector at answer-build time; the digest cache stores
only hybrid answers, never a provisional bm25 one. The background worker batches
through a Dixie gate and WaitBackground polls for true idle.
Co-Authored-By: ornith-1.5-35b-a3b <ornith-1.5-35b-a3b@llama.cpp.invalid>
