Add router-on-a-stick, commit-confirm rollback and an example config

- Allow the WAN link to also carry LANs when the uplink is PPPoE on a
  VLAN with IPv6 off; only the PPP device counts as an uplink then, so
  LAN traffic can never match WAN rules.
- Add router.rollback: router-arm / router-confirm / router-rollback
  plus `nix run .#deploy`, a commit-confirm deployment flow.
- Wait for the LAN devices instead of the uplink in networkd
  wait-online, so an unplugged uplink cannot block boot or activation.
- Add examples/desktop (single NIC, PPPoE on VLAN 1001) as
  nixosConfigurations.example, built as a flake check.

Covered by the onestick and rollback VM tests and new assertion cases.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

modules/router/options.nix

@@ -14,6 +14,10 @@ in
         description = ''
           Physical uplink interface. Native IPv6 (DHCPv6) runs on it untagged;
           with PPPoE it also carries the (optionally tagged) PPPoE session.
+
+          The link may also carry LANs ("router on a stick") when the uplink
+          is PPPoE on a VLAN and IPv6 is disabled: the untagged link is then
+          free for a LAN and only the PPP device counts as an uplink.
         '';
       };