Moves the three incus_network resources into a dedicated module called
once per entry in var.incus_networks. State was migrated imperatively
via `terraform state mv` because lxc/incus v1.0.2 does not implement
ResourceWithMoveState, making declarative moved blocks unsupported.
Plan confirms zero creates/destroys after migration.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
