feat(zfs): import ZFS pools and add the shared write group

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.

base.nix

@@ -62,6 +62,12 @@ in
       extraGroups = [ "wheel" ];
     };
 
+    # The shared write group.
+    # Its gid is fixed, so a host and every guest carry the same number.
+    # An identity-mapped container write then lands on the pool as this group, sparing every service the permission juggling.
+    # 10000 clears the system-group ids assigned automatically and leaves headroom above the primary user, so nothing else claims it.
+    users.groups.storage.gid = 10000;
+
     # home-manager as a NixOS module: one build produces the system and user
     # environment together, sharing the system's pkgs and installing user
     # packages into the system profile.