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/tests/ipv6_test.rs

@@ -38,7 +38,7 @@ async fn test_route_peer_info_ipv6() {
     global_ctx.set_ipv6(Some(ipv6_cidr));
 
     // Create RoutePeerInfo with IPv6 support
-    let updated_info = RoutePeerInfo::new_updated_self(123, 456, &global_ctx);
+    let updated_info = RoutePeerInfo::new_updated_self(123, 456, &global_ctx, None);
 
     // Verify IPv6 address is included
     assert!(updated_info.ipv6_addr.is_some());