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/rpc_service.rs

@@ -13,9 +13,9 @@ use crate::{
             GetWhitelistRequest, GetWhitelistResponse, ListCredentialsRequest,
             ListCredentialsResponse, ListForeignNetworkRequest, ListForeignNetworkResponse,
             ListGlobalForeignNetworkRequest, ListGlobalForeignNetworkResponse, ListPeerRequest,
-            ListPeerResponse, ListRouteRequest, ListRouteResponse, PeerInfo, PeerManageRpc,
-            RevokeCredentialRequest, RevokeCredentialResponse, ShowNodeInfoRequest,
-            ShowNodeInfoResponse,
+            ListPeerResponse, ListPublicIpv6InfoRequest, ListPublicIpv6InfoResponse,
+            ListRouteRequest, ListRouteResponse, PeerInfo, PeerManageRpc, RevokeCredentialRequest,
+            RevokeCredentialResponse, ShowNodeInfoRequest, ShowNodeInfoResponse,
         },
         rpc_types::{self, controller::BaseController},
     },
@@ -99,6 +99,16 @@ impl PeerManageRpc for PeerManagerRpcService {
         Ok(reply)
     }
 
+    async fn list_public_ipv6_info(
+        &self,
+        _: BaseController,
+        _request: ListPublicIpv6InfoRequest,
+    ) -> Result<ListPublicIpv6InfoResponse, rpc_types::error::Error> {
+        Ok(weak_upgrade(&self.peer_manager)?
+            .get_local_public_ipv6_info()
+            .await)
+    }
+
     async fn list_route(
         &self,
         _: BaseController,