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_manager.rs

@@ -1291,6 +1291,14 @@ impl PeerManager {
         self.get_route().list_proxy_cidrs_v6().await
     }
 
+    pub async fn list_public_ipv6_routes(&self) -> BTreeSet<cidr::Ipv6Inet> {
+        self.get_route().list_public_ipv6_routes().await
+    }
+
+    pub async fn get_my_public_ipv6_addr(&self) -> Option<cidr::Ipv6Inet> {
+        self.get_route().get_my_public_ipv6_addr().await
+    }
+
     pub async fn dump_route(&self) -> String {
         self.get_route().dump().await
     }
@@ -1879,6 +1887,7 @@ impl PeerManager {
             version: EASYTIER_VERSION.to_string(),
             feature_flag: Some(self.global_ctx.get_feature_flags()),
             ip_list: Some(self.global_ctx.get_ip_collector().collect_ip_addrs().await),
+            public_ipv6_addr: self.get_my_public_ipv6_addr().await.map(Into::into),
         }
     }