feat: support allocating public IPv6 addresses from a provider (#2162)

* feat: support allocating public IPv6 addresses from a provider

Add a provider/leaser architecture for public IPv6 address allocation
between nodes in the same network:

- A node with `--ipv6-public-addr-provider` advertises a delegable
  public IPv6 prefix (auto-detected from kernel routes or manually
  configured via `--ipv6-public-addr-prefix`).
- Other nodes with `--ipv6-public-addr-auto` request a /128 lease from
  the selected provider via a new RPC service (PublicIpv6AddrRpc).
- Leases have a 30s TTL, renewed every 10s by the client routine.
- The provider allocates addresses deterministically from its prefix
  using instance-UUID-based hashing to prefer stable assignments.
- Routes to peer leases are installed on the TUN device, and each
  client's own /128 is assigned as its IPv6 address.

Also includes netlink IPv6 route table inspection, integration tests,
and event-driven route/address reconciliation.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

easytier/src/instance_manager.rs

@@ -435,6 +435,20 @@ fn handle_event(
                         event!(info, ?ip, "[{}] dhcp ip conflict", instance_id);
                     }
 
+                    GlobalCtxEvent::PublicIpv6Changed(old, new) => {
+                        event!(info, ?old, ?new, "[{}] public ipv6 changed", instance_id);
+                    }
+
+                    GlobalCtxEvent::PublicIpv6RoutesUpdated(added, removed) => {
+                        event!(
+                            info,
+                            ?added,
+                            ?removed,
+                            "[{}] public ipv6 routes updated",
+                            instance_id
+                        );
+                    }
+
                     GlobalCtxEvent::PortForwardAdded(cfg) => {
                         event!(
                             info,