skydick: extract datapool.nix for Mach2 ZFS storage config ...

Move all storage-serving config (NFS, Samba, iSCSI, tmpfiles, firewall
ports, storage group) from default.nix into datapool.nix. Add Mach2
dual-actuator mirror layout documentation, new datasets (torrent, vm),
and clean permission model (setgid storage group for user data).

Co-Authored-By: Claude Opus 4.6 <[email protected]>