`sessions_for` recorded what opened from `gather`'s results, which arrive only
when it runs to completion. Cancelling it discarded them, so a database that
opened while a sibling was still pending was never recorded and `aclose` never
closed it. `return_exceptions=True` covers a failing child, not a cancelled
parent.
`_open` now registers its own session, so what opened is reachable however the
fan-out ends.
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/`.
