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/core.rs

@@ -171,6 +171,31 @@ struct NetworkOptions {
     )]
     ipv6: Option<String>,
 
+    #[arg(
+        long,
+        env = "ET_IPV6_PUBLIC_ADDR_PROVIDER",
+        help = t!("core_clap.ipv6_public_addr_provider").to_string(),
+        num_args = 0..=1,
+        default_missing_value = "true"
+    )]
+    ipv6_public_addr_provider: Option<bool>,
+
+    #[arg(
+        long,
+        env = "ET_IPV6_PUBLIC_ADDR_AUTO",
+        help = t!("core_clap.ipv6_public_addr_auto").to_string(),
+        num_args = 0..=1,
+        default_missing_value = "true"
+    )]
+    ipv6_public_addr_auto: Option<bool>,
+
+    #[arg(
+        long,
+        env = "ET_IPV6_PUBLIC_ADDR_PREFIX",
+        help = t!("core_clap.ipv6_public_addr_prefix").to_string()
+    )]
+    ipv6_public_addr_prefix: Option<String>,
+
     #[arg(
         short,
         long,
@@ -875,6 +900,20 @@ impl NetworkOptions {
             })?))
         }
 
+        if let Some(enabled) = self.ipv6_public_addr_provider {
+            cfg.set_ipv6_public_addr_provider(enabled);
+        }
+
+        if let Some(enabled) = self.ipv6_public_addr_auto {
+            cfg.set_ipv6_public_addr_auto(enabled);
+        }
+
+        if let Some(prefix) = &self.ipv6_public_addr_prefix {
+            cfg.set_ipv6_public_addr_prefix(Some(prefix.parse().with_context(|| {
+                format!("failed to parse ipv6 public address prefix: {}", prefix)
+            })?));
+        }
+
         if !self.peers.is_empty() {
             let mut peers = cfg.get_peers();
             peers.reserve(peers.len() + self.peers.len());