Open a location as a zarr group of a KNOWN format, or by
openGroupPreferV3 when format is null.
For probing an optional node on a raw store (no consolidated index to answer
from): a format guess costs one 404 per wrong document — up to three for an
absent node — where a known format costs one. Format 2 checks .zgroup
before open.v2 reads .zattrs (which a group without attributes lacks), so
an absent group is ONE request.
Open a location as a zarr group of a KNOWN format, or by openGroupPreferV3 when
formatisnull.For probing an optional node on a raw store (no consolidated index to answer from): a format guess costs one 404 per wrong document — up to three for an absent node — where a known format costs one. Format 2 checks
.zgroupbeforeopen.v2reads.zattrs(which a group without attributes lacks), so an absent group is ONE request.