feat(cli): select transport in config; server MultiServer + client dial handover

- aura-cli config gains [transport] (order + per-transport ports + obfuscate/
  masquerade); server binds all enabled transports via MultiServer, client uses
  dial() with UDP->TCP->QUIC handover. Config examples updated; backward-compatible
  (defaults to udp,tcp,quic). 21 cli tests incl. a real-UDP-transport loopback.
- docs/sing-box.md: integration approach note (process-bridge now; native Go
  outbound for phones, with crypto-library mapping + KAT requirement).
- Normalize rustfmt across the v2 transport files (tcp/dial/udp contract).

Whole workspace: 97 tests pass, clippy -D warnings clean, fmt clean. Deploy flow
(pki init/issue-server/issue-client) validated with the release binary.

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

crates/aura-transport/src/tcp.rs

@@ -109,7 +109,11 @@ impl PacketConnection for TcpConnection {
                 Frame::Data { payload, .. } => return Ok(payload.to_vec()),
                 Frame::Ping { seq } => {
                     // Separate mutex from the receive lock we hold => no deadlock.
-                    self.sender.lock().await.send_frame(Frame::Pong { seq }).await?;
+                    self.sender
+                        .lock()
+                        .await
+                        .send_frame(Frame::Pong { seq })
+                        .await?;
                 }
                 Frame::Pong { .. } => continue,
                 Frame::Close { code, reason } => {