labsim: add IPv6 BGP config layer for the Gateway-API public-v6 rehearsal

The sim's eBGP was IPv4-only; extend it to the v6 family so labsim can rehearse
Cilium BGP advertising a public-style v6 LoadBalancer /64 to the VyOS pair before
it touches production (approved plan: public IPv6 via Cilium BGP + Gateway API).

- sim-net-config.py: bgp6 inside bgp() -- ipv6-unicast peer-group K8S6 over
  fd00:2::11/12/13, prefix-list6 (le 128, /128 host routes), K8S-IN6/OUT6 route-maps
  (export deny -- never hand the cluster a default). ULA fd61:1e00::/64 as the sim
  LB range so nothing leaks into the real HE /48. All v6 lines pass the
  sim-net-apply.sh whitelist.
- sim-ha-config.py: VLANS6 = {2: fd00:2/64} -> routers hold fd00:2::252/253 on
  bond0.2 so they can peer the nodes over v6 (no v6 VRRP VIP; BGP peers the per-box
  address, as production).
- k8s-up.sh: nodes get fd00:2::11/12/13 + dual-stack k3s (dual node-ip, dual
  cluster/service CIDR). Peers line up with sim-net-config K8S_NODES_V6.

Config-gen verified for all three. Live chunk (Cilium v6+BGP+Gateway install,
bring-up, datapath/#26847 proof) is next.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01DMVzWZgiKW2wquf5z8S1yH

labsim/sim-ha-config.py

@@ -45,6 +45,12 @@ VLANS = {
     10:  ("172.31.10",  23),
     200: ("172.31.200", 24),
 }
+# VLAN 2 (k8s) also gets a ULA IPv6, so the routers can peer eBGP with the nodes
+# over IPv6 (the neighbors in sim-net-config.py K8S_NODES_V6 = fd00:2::1x). Only
+# VLAN 2 needs it for the BGP rehearsal; a ULA keeps sim traffic out of the real
+# HE /48. Routers hold ::252 / ::253 (no v6 VRRP VIP -- BGP peers the real per-box
+# address, exactly as production).
+VLANS6 = {2: ("fd00:2", 64)}
 DHCP_HA_NAME = "labsim-dhcp-pair"     # must not equal either host-name
 
 
@@ -73,6 +79,8 @@ def build(role: str) -> list[str]:
             # .1 becomes the floating VIP, exactly as production will be.
             f"delete interfaces bonding {iface} address",
             f"set interfaces bonding {iface} address '{pfx}.{self_o}/{cidr}'",
+            *([f"set interfaces bonding {iface} address '{VLANS6[vlan][0]}::{self_o}/{VLANS6[vlan][1]}'"]
+              if vlan in VLANS6 else []),
             f"set high-availability vrrp group {g} interface bond0.{vlan}",
             f"set high-availability vrrp group {g} vrid {vlan}",
             f"set high-availability vrrp group {g} address {pfx}.1/{cidr}",