Migrating from TrueNAS to NixOS without losing data after the build-system rug pull

TrueNAS was the last non-NixOS machine in my home lab. I kept postponing it for a simpler reason: moving something with ~100 TiB of data feels scary. My NAS holds the big ZFS pools. It has the SMB shares, NFS exports, snapshots, replication tasks, encrypted datasets, Incus state, Docker workloads, and all the boring-but-important health checks that are easy to take for granted until something…