Clients on stellio's lan had no global IPv6. AIS delegates a single /64
over PPPoE; cnx-network's router service carves the VLAN prefixes with
SubnetId=auto, so vlan-mgmt took the whole prefix and vlan-lan logged
"Couldn't find a suitable prefix. Ran out of address space" -- no RA
prefix on lan.
Pin SubnetId: 0x0 on vlan-lan (the whole /64), 0x1 on vlan-mgmt (out of
range on a /64, harmless; works unchanged if AIS ever delegates wider).
Verified live via runtime drop-ins: vlan-lan got 2405:9800:b910:1f5d::/64
and a LAN client pinged 2606:4700:4700::1111 over v6.
Drop once cnx-network carries the per-VLAN `ipv6SubnetId` option
(cnx-network-clan commit 27c697a) and the input is bumped.