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/lib.rs

@@ -68,5 +68,5 @@ pub use config::{
     ForwardingRule, RateLimitConfig, StaticConfig,
 };
 pub use error::{AuthError, ChannelError, ConfigError, ForwardError, TransportError};
-pub use server::serve::{ServeError, ServeOptions, ServeTransportMode, Server};
+pub use server::serve::{ListenerConfig, ServeError, ServeOptions, ServeTransportMode, Server};
 pub use transport::{Transport, TransportAcceptor, TransportInfo, TransportKind};