bond0 (ConnectX-4 LX 25G, active-backup) was carrying 10.0.1.1/16
until the 2026-05-15 cutover onto bond40g (2× 40G ConnectX-3 LACP
layer3+4, MTU 9200). With cutover done and verified — Aggregator ID
1 on both slaves, jumbo end-to-end to gateway, traffic flowing —
the old bond is dead weight.

* Remove the `bonds.bond0` and `interfaces.bond0` blocks.
* Rename the remaining active `bond0` references to `bond40g`:
  - `systemd.network.networks."40-bond0"` → `."40-bond40g"`
  - `"net.ipv6.conf.bond0.accept_ra"` sysctl
  - `skyworks.monitoring.netInterfaces = [ "bond0" ]`
  - wait-online and RA-leak comments
* The freed enp4s0f0np0/enp4s0f1np1 are now standalone DOWN,
  available for future use.

The live kernel `bond0` device persisted past nixos-rebuild
because networkd doesn't destroy unmanaged ifaces; cleaned up
manually with `ip link set <slave> nomaster; ip link del bond0`.