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_config.proto

@@ -24,6 +24,9 @@ message InstanceConfigPatch {
   repeated ExitNodePatch exit_nodes = 8;
   repeated UrlPatch mapped_listeners = 9;
   repeated UrlPatch connectors = 10;
+  optional bool ipv6_public_addr_provider = 11;
+  optional bool ipv6_public_addr_auto = 12;
+  optional string ipv6_public_addr_prefix = 13;
 }
 
 message PortForwardPatch {