The documented rule was existence in more than one selected database. The rule
is narrower and does not need to be wider: a copy the search never returned
grounded nothing, so the retrieved result is the citation and its database is
recorded rather than guessed.
The page now states the whole rule, including the fallback's own check for an
id no search returned. Two retrieved copies are refused, and so is an id
already cited from another database in the conversation. The case between them
had no test.
Collision detection replaced the lookup's dict comprehension with
`setdefault`, which also flipped a chunk found by several searches from its
last occurrence to its first. The copies differ in everything the expansion
window decides, figures included, so that silently changed what a citation
renders.
The rules are separate now: a repeated (source, chunk_id) takes the later
result, a chunk_id under two sources is still refused.
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/`.
