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/connector/udp_hole_punch/common.rs

@@ -720,7 +720,7 @@ async fn check_udp_socket_local_addr(
     let socket = UdpSocket::bind("0.0.0.0:0").await?;
     socket.connect(remote_mapped_addr).await?;
     if let Ok(local_addr) = socket.local_addr() {
-        // local_addr should not be equal to virtual ipv4 or virtual ipv6
+        // local_addr should not be equal to an EasyTier-managed virtual/public address.
         match local_addr.ip() {
             IpAddr::V4(ip) => {
                 if global_ctx.get_ipv4().map(|ip| ip.address()) == Some(ip) {
@@ -728,8 +728,8 @@ async fn check_udp_socket_local_addr(
                 }
             }
             IpAddr::V6(ip) => {
-                if global_ctx.get_ipv6().map(|ip| ip.address()) == Some(ip) {
-                    return Err(anyhow::anyhow!("local address is virtual ipv6").into());
+                if global_ctx.is_ip_local_ipv6(&ip) {
+                    return Err(anyhow::anyhow!("local address is easytier-managed ipv6").into());
                 }
             }
         }