truenas-migration: mdadm boot mirror, clean zfs export step

Use TrueNAS boot-pool SSDs as mdadm RAID1 for NixOS root to keep
the boot path ZFS-independent. Added zfs export step before shutdown.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

docs/plans/truenas-migration.md

@@ -92,8 +92,10 @@ Expand storage capacity for the main hdd-pool. Since we need to add disks anyway
    - Configure monitoring (node-exporter, promtail, smartctl-exporter)
 
 3. **Install NixOS**:
-   - Install NixOS on boot drive (SSD/NVMe, separate from ZFS pool)
-   - Import existing ZFS pool
+   - `zfs export hdd-pool` on TrueNAS before shutdown (clean export)
+   - Wipe TrueNAS boot-pool SSDs, set up as mdadm RAID1 for NixOS root
+   - Install NixOS on mdadm mirror (keeps boot path ZFS-independent)
+   - Import hdd-pool via `boot.zfs.extraPools`
    - Verify all datasets mount correctly
 
 4. **Service migration**:
@@ -148,7 +150,7 @@ will function fine with uneven distribution, just slightly suboptimal for perfor
 
 - [ ] Hostname for new NAS host? (nas1? storage1?)
 - [ ] IP address allocation (keep 10.69.12.50 or new IP?)
-- [ ] Boot drive: which SSD/NVMe for NixOS root?
+- [x] Boot drive: Reuse TrueNAS boot-pool SSDs as mdadm RAID1 for NixOS root (no ZFS on boot path)
 - [ ] Retire old 8TB drives? (SMART looks healthy, keep unless chassis space is needed)
 - [ ] Drive trays: do new 24TB drives fit, or order trays from abroad?
 - [ ] Timeline/maintenance window for NixOS swap?