- 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