feat(core): implement multi-transport listeners with ListenerConfig and Vec<ListenerConfig>

- Add ListenerConfig struct with transport_kind, listen_addr, per-transport config
- Add Dns and WebTransport variants to TransportKind (tags only, no behavior)
- Add .listeners() builder method to ServeOptions for multi-listener config
- Keep .transport_mode() backwards compatible (creates single-element listeners vec)
- Update Server::run() to use listeners from Server struct (first listener)
- Add Server::listeners() accessor for multi-transport listener configs
- Update StaticConfig to support listeners field, converted from ServeOptions
- All listeners share Arc<ArcSwap<DynamicConfig>>, ConnectionRateLimiter, and IdentityProvider
- Graceful shutdown terminates accept loop via existing shutdown signal
- TOML [[listeners]] array-of-tables syntax supported via ListenerConfig in StaticConfig
- Add comprehensive tests for ListenerConfig, multi-listener ServeOptions, Server creation

crates/alknet-core/src/transport/mod.rs

@@ -91,6 +91,8 @@ pub enum TransportKind {
     Tcp,
     Tls { server_name: Option<String> },
     Iroh { endpoint_id: String },
+    Dns { domain: String },
+    WebTransport { host: String },
 }
 
 #[cfg(test)]
@@ -169,6 +171,12 @@ mod tests {
         let iroh = TransportKind::Iroh {
             endpoint_id: "abc123".to_string(),
         };
+        let dns = TransportKind::Dns {
+            domain: "example.com".to_string(),
+        };
+        let wt = TransportKind::WebTransport {
+            host: "example.com".to_string(),
+        };
 
         if let TransportKind::Tcp = tcp {}
         if let TransportKind::Tls {
@@ -180,5 +188,11 @@ mod tests {
         if let TransportKind::Iroh { endpoint_id } = iroh {
             assert_eq!(endpoint_id, "abc123");
         }
+        if let TransportKind::Dns { domain } = dns {
+            assert_eq!(domain, "example.com");
+        }
+        if let TransportKind::WebTransport { host } = wt {
+            assert_eq!(host, "example.com");
+        }
     }
 }