Add `modules.zfs`, the host-level pool import: a host declares its ZFS host id
and the pools to import with their dataset mountpoints, and the module imports
those pools as durable state rather than recreating them, so a service's data
survives any rebuild or reimage.
Add a shared `storage` group with a fixed gid to the base config both the host
base and the guest-base build on, so a host and every guest carry the same
number and an identity-mapped container write lands on the pool as that group
without per-service permission juggling.
No host enables the module: the only host is a laptop with no pools and a
kernel with no ZFS build, so the enabled build was verified by ad-hoc
enablement against a ZFS-supported kernel while the committed tree stays inert.
