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>

Cargo.lock

@@ -263,6 +263,7 @@ name = "aura-transport"
 version = "0.1.0"
 dependencies = [
  "anyhow",
+ "async-trait",
  "aura-crypto",
  "aura-proto",
  "bytes",