router: pin stellio mgmt ipv6SubnetId to 1

Restores the mgmt subnet id the dropped mkForce used, via cnx-network's
new per-VLAN option instead of overriding networkd config. Still skipped
on AIS's bare /64; unlike the VLAN-id default (10) it also fits a /61-/63
delegation.

inventories/router.nix

@@ -82,8 +82,14 @@ in
       };
 
       # 10.1.<id>.0/24, router .1, DHCP pool .100-.199.
+      # ipv6SubnetId: which /64 of the DHCPv6-PD prefix the VLAN announces.
+      # lan defaults to 0 (whole prefix on AIS's bare /64). mgmt would default
+      # to its VLAN id (10); keep it at 1 so it also fits a /61-/63 delegation.
       vlans = {
-        mgmt.id = 10;
+        mgmt = {
+          id = 10;
+          ipv6SubnetId = 1;
+        };
         lan.id = 20;
       };
     };