`sources=["alpha"]` on a client covering a set went through fusion, which
scores position: a result the database ranked at 0.6549 was reported as
1/(60+rank). Embedding also moved ahead of the repository, so a filter matching
no document embedded the query anyway.
A selection of one now runs the single-database search. Fusion reconciles
rankings from separate indexes, and one ranking has nothing to reconcile.
A reranker that returns chunks it built rather than the ones it was given loses
which database each came from, since ownership is by identity. That is named
now instead of surfacing as a KeyError, and stated on `RerankerBase._rerank`.
Two files of 1,405 and 814 lines become seven: scope resolution, lifecycle,
search, documents, expansion, citations and capabilities. `_config`, `_seed`
and the rest move to `helpers.py`, importable by the sandbox tests that share
them, and the package points VCR back at `tests/cassettes/multi_db/`.
