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/peer_rpc.proto

@@ -47,6 +47,9 @@ message RoutePeerInfo {
 
   // Trusted credential public keys published by admin nodes (holding network_secret)
   repeated TrustedCredentialPubkeyProof trusted_credential_pubkeys = 19;
+
+  optional common.Ipv6Inet ipv6_public_addr_prefix = 22;
+  optional common.Ipv6Inet ipv6_public_addr_lease = 24;
 }
 
 message PeerIdVersion {
@@ -133,6 +136,46 @@ service OspfRouteRpc {
   rpc SyncRouteInfo(SyncRouteInfoRequest) returns (SyncRouteInfoResponse);
 }
 
+message AcquireIpv6PublicAddrLeaseRequest {
+  uint32 peer_id = 1;
+  common.UUID inst_id = 2;
+}
+
+message RenewIpv6PublicAddrLeaseRequest {
+  uint32 peer_id = 1;
+  common.UUID inst_id = 2;
+  common.Ipv6Inet leased_addr = 3;
+}
+
+message ReleaseIpv6PublicAddrLeaseRequest {
+  uint32 peer_id = 1;
+  common.UUID inst_id = 2;
+}
+
+message GetIpv6PublicAddrLeaseRequest {
+  uint32 peer_id = 1;
+  common.UUID inst_id = 2;
+}
+
+message Ipv6PublicAddrLeaseReply {
+  uint32 provider_peer_id = 1;
+  common.UUID provider_inst_id = 2;
+  common.Ipv6Inet provider_prefix = 3;
+  common.Ipv6Inet leased_addr = 4;
+  google.protobuf.Timestamp valid_until = 5;
+  bool reused = 6;
+  optional string error_msg = 7;
+}
+
+service PublicIpv6AddrRpc {
+  rpc AcquireLease(AcquireIpv6PublicAddrLeaseRequest)
+      returns (Ipv6PublicAddrLeaseReply);
+  rpc RenewLease(RenewIpv6PublicAddrLeaseRequest)
+      returns (Ipv6PublicAddrLeaseReply);
+  rpc ReleaseLease(ReleaseIpv6PublicAddrLeaseRequest) returns (common.Void);
+  rpc GetLease(GetIpv6PublicAddrLeaseRequest) returns (Ipv6PublicAddrLeaseReply);
+}
+
 message GetIpListRequest {}
 
 message GetIpListResponse {