fix: persist veth-wan pair across reboot (mulga-998.b Fix 1)

setup-ovn.sh's veth branch creates veth-wan-br ↔ veth-wan-ovs but nothing
recreates the kernel interfaces on reboot. OVSDB remembers veth-wan-ovs
as a port on br-ext but the peer is gone — the external path silently
breaks and vpcd falls through to direct mode.

- setup-ovn.sh: write /etc/systemd/network/15-spinifex-veth-wan.{netdev,network}
  in the veth branch so systemd-networkd recreates the pair on boot. Add
  a single pre-case guard that rips down stale veth persistence + the OVS
  port + the kernel veth whenever WAN_BRIDGE_MODE != veth, so mode
  transitions are idempotent (D17).
- reset-dev-env.sh: delete the netdev/network units during cleanup so
  `make reset` gives a truly clean slate.
- spinifex-vpcd.service: After=systemd-networkd.service openvswitch-switch.service
  so vpcd's sanity checks see a populated kernel + OVSDB (D5).

scripts/reset-dev-env.sh

@@ -107,6 +107,16 @@ if ip link show veth-wan-br >/dev/null 2>&1; then
     sudo ip link del veth-wan-br 2>/dev/null || true
 fi
 
+# Remove veth persistence units (Fix 1, mulga-998.b). Without this, systemd-networkd
+# recreates the veth on next reboot even after a full dev reset.
+if [ -e /etc/systemd/network/15-spinifex-veth-wan.netdev ] || \
+   [ -e /etc/systemd/network/15-spinifex-veth-wan.network ]; then
+    echo "  Deleting veth persistence units"
+    sudo rm -f /etc/systemd/network/15-spinifex-veth-wan.netdev \
+               /etc/systemd/network/15-spinifex-veth-wan.network
+    sudo networkctl reload 2>/dev/null || true
+fi
+
 # Remove macvlan interfaces created by setup-ovn.sh
 for iface in $(ip -o link show type macvlan 2>/dev/null | awk -F': ' '{print $2}' | grep '^spx-ext-'); do
     echo "  Deleting macvlan: $iface"