`resolve_document` and `find_document` selected a document through a listing,
then dropped its source and looked the id up across the set. Ids repeat between
copies of a database, so a title that matched in one could be answered by
another's document.
`get_document_by_id` and `get_chunk_by_id` join `get_picture_bytes` in taking an
optional `source`, and all three route it through `clients_covering`, so a name
the client does not cover raises `UnknownDatabaseError` rather than being
answered by the database it does cover. Without a source the reads are as they
were, answering from the first database in configured order that holds the id.
Reads tagged the document they returned and writes did not, so creating in
`alpha` came back with `source=None` while reading the same document came back
with `"alpha"`. Every outward write result goes through the owning session.
`--database` and `--db` read as one word abbreviated but take different
things, a configured name against a filesystem path, and they sit in
different positions: `haiku-rag --db /path info` fails with `No such option`.
The CLI already says "database" to the user everywhere it prints one, so the
name it selects by should say the same. Unreleased, so no deprecation.
Also: without a reranker, raise `search.limit` with the number of databases
searched, since each contributes its best matches to a list truncated back
to the limit.
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/`.
