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/peers/peer_rpc_service.rs

@@ -41,6 +41,10 @@ impl DirectConnectorRpc for DirectConnectorManagerRpcServer {
             let et_ipv6: crate::proto::common::Ipv6Addr = et_ipv6.address().into();
             ret.interface_ipv6s.retain(|x| *x != et_ipv6);
         }
+        if let Some(public_ipv6) = self.global_ctx.get_public_ipv6_lease() {
+            let public_ipv6: crate::proto::common::Ipv6Addr = public_ipv6.address().into();
+            ret.interface_ipv6s.retain(|x| *x != public_ipv6);
+        }
         tracing::trace!(
             "get_ip_list: public_ipv4: {:?}, public_ipv6: {:?}, listeners: {:?}",
             ret.public_ipv4,