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/common/ifcfg/mod.rs

@@ -166,3 +166,14 @@ pub type IfConfiger = DummyIfConfiger;
 
 #[cfg(target_os = "windows")]
 pub use windows::RegistryManager;
+
+#[cfg(target_os = "linux")]
+pub(crate) fn list_ipv6_route_messages()
+-> Result<Vec<netlink_packet_route::route::RouteMessage>, Error> {
+    netlink::NetlinkIfConfiger::list_ipv6_route_messages()
+}
+
+#[cfg(target_os = "linux")]
+pub(crate) fn get_interface_index(name: &str) -> Result<u32, Error> {
+    netlink::NetlinkIfConfiger::get_interface_index(name)
+}