Opening a database ran `list_tables` three times, opened the settings table
twice, and read and parsed the same settings row three times: once for the stored
vector dimension, once for the version behind the migration check, and once for
config validation. On object storage each of those is a round trip.
`_initialize` now reads both once and threads them down. `_init_tables` and
`_check_migrations` take what it read instead of fetching their own copy, and
`validate_config_compatibility` accepts the settings it should compare against,
still reading for itself when called directly.
Passing the pre-init read to validation is equivalent: nothing between the read
and the validation rewrites `embeddings`, which is all it compares.
The settings read no longer swallows every exception. It did before, when the
only consequence was falling back to the configured vector dimension; now the
same empty result feeds the migration check, where it would read as version
0.0.0 and declare every migration pending. Only decode failures are tolerated,
and a decoded non-object normalizes to {} rather than reaching callers that
expect a mapping.
