Add zfs_pool_mode: file|single|mirror.
- file (default): loopback disk image at zfs_pool_file_path — real ZFS with
no spare disk, ideal for a cost-optimized test VM; wipes nothing.
- single/mirror: whole spare disk(s); mirror for production redundancy.
Guard/probe now loops over zfs_pool_disks. Update group_vars, README (modes,
prerequisites, safety), and CLAUDE.md current-focus note.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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