feat(transport): TCP/443 fallback + unified dialer with UDP->TCP->QUIC handover

- tcp.rs: Aura proto handshake + Session directly over TcpStream (TcpServer/
  TcpClient/TcpConnection: PacketConnection), with an optional light HTTP/1.1
  masquerade preamble. Fallback for UDP-blocking networks. (Full TLS-443 mimicry
  is a documented follow-up.)
- dial.rs: TransportMode {Udp,Tcp,Quic}, Endpoints, DialConfig; client `dial()`
  tries transports in order and hands over on failure/timeout; MultiServer binds
  and accepts on every enabled transport at once (TCP/QUIC multi-client; UDP
  single-peer-per-accept in v1).
- Tests: tcp loopback (plain + masquerade), dial handover (dead TCP -> UDP).
  clippy/fmt clean.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

crates/aura-transport/src/lib.rs

@@ -64,15 +64,19 @@
 #![warn(missing_docs)]
 
 pub mod conn;
+pub mod dial;
 pub mod mimicry;
 pub mod padding;
 pub mod quic;
+pub mod tcp;
 pub mod udp;
 
 pub use conn::AuraConnection;
+pub use dial::{dial, Accepted, DialConfig, Endpoints, MultiServer, TransportMode};
 pub use mimicry::{alpn_protocols, chrome_quic_transport_config, ALPN_H3, DEFAULT_SNI};
 pub use padding::{inject_padding_frames, pad_to_https_size, HTTPS_SIZE_BUCKETS};
 pub use quic::{client_endpoint, server_endpoint, AcceptAnyServerCert};
+pub use tcp::{TcpClient, TcpConnection, TcpOpts, TcpServer};
 pub use udp::{UdpClient, UdpConnection, UdpOpts, UdpServer};
 
 // Re-export the inner proto trait so downstream crates (the CLI) can name the connection as