feat(transport): pin PacketConnection contract for the router seam

Define the async PacketConnection trait (send_packet/recv_packet over &self)
that aura-tunnel's router consumes and the QUIC connection will implement.
Committed before Wave 3 so the transport and tunnel agents build against a
stable cross-crate contract from isolated worktrees.

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

crates/aura-transport/src/conn.rs

@@ -0,0 +1,20 @@
+//! The transport's full-duplex packet-connection abstraction.
+//!
+//! [`PacketConnection`] is the seam between this crate (QUIC transport + the post-handshake
+//! `aura_proto::Session`) and `aura-tunnel`'s router. The router reads IP packets from the TUN
+//! device and `send_packet`s the ones routed through the VPN, while a second task `recv_packet`s
+//! decrypted IP packets to write back to the TUN. The methods take `&self` (not `&mut self`) so a
+//! single connection can be shared — e.g. behind `Arc<dyn PacketConnection>` — across the
+//! concurrent send and receive tasks.
+
+use async_trait::async_trait;
+
+/// A bidirectional, encrypted packet pipe to the peer (one IP packet per call).
+#[async_trait]
+pub trait PacketConnection: Send + Sync {
+    /// Encrypt and send one IP packet to the peer.
+    async fn send_packet(&self, packet: &[u8]) -> anyhow::Result<()>;
+
+    /// Receive and decrypt one IP packet from the peer. Returns the plaintext IP packet.
+    async fn recv_packet(&self) -> anyhow::Result<Vec<u8>>;
+}

crates/aura-transport/src/lib.rs

@@ -1 +1,9 @@
-//! aura-transport — QUIC transport and traffic mimicry (skeleton; implemented in Wave 3).
+//! aura-transport — QUIC transport, HTTPS/H3 traffic mimicry, and the packet-connection seam.
+//!
+//! Implemented in Wave 3. This file currently pins the cross-crate [`PacketConnection`] contract
+//! consumed by `aura-tunnel`'s router; the QUIC endpoint (quinn), mimicry, and padding land
+//! alongside it in the `quic`, `mimicry`, and `padding` modules.
+
+pub mod conn;
+
+pub use conn::PacketConnection;