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/proto/api_instance.proto

@@ -81,6 +81,8 @@ message Route {
   optional int32 path_latency_latency_first = 14;
 
   common.Ipv6Inet ipv6_addr = 15;
+  common.Ipv6Inet public_ipv6_addr = 16;
+  common.Ipv6Inet ipv6_public_addr_prefix = 17;
 }
 
 message PeerRoutePair {
@@ -100,12 +102,29 @@ message NodeInfo {
   string version = 9;
   common.PeerFeatureFlag feature_flag = 10;
   peer_rpc.GetIpListResponse ip_list = 11;
+  common.Ipv6Inet public_ipv6_addr = 12;
+  common.Ipv6Inet ipv6_public_addr_prefix = 13;
 }
 
 message ShowNodeInfoRequest { InstanceIdentifier instance = 1; }
 
 message ShowNodeInfoResponse { NodeInfo node_info = 1; }
 
+message PublicIpv6LeaseInfo {
+  uint32 peer_id = 1;
+  string inst_id = 2;
+  common.Ipv6Inet leased_addr = 3;
+  int64 valid_until_unix_seconds = 4;
+  bool reused = 5;
+}
+
+message ListPublicIpv6InfoRequest { InstanceIdentifier instance = 1; }
+
+message ListPublicIpv6InfoResponse {
+  common.Ipv6Inet provider_prefix = 1;
+  repeated PublicIpv6LeaseInfo provider_leases = 2;
+}
+
 message ListRouteRequest { InstanceIdentifier instance = 1; }
 
 message ListRouteResponse { repeated Route routes = 1; }
@@ -167,6 +186,8 @@ message GetForeignNetworkSummaryResponse {
 
 service PeerManageRpc {
   rpc ListPeer(ListPeerRequest) returns (ListPeerResponse);
+  rpc ListPublicIpv6Info(ListPublicIpv6InfoRequest)
+      returns (ListPublicIpv6InfoResponse);
   rpc ListRoute(ListRouteRequest) returns (ListRouteResponse);
   rpc DumpRoute(DumpRouteRequest) returns (DumpRouteResponse);
   rpc ListForeignNetwork(ListForeignNetworkRequest)