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.toml

@@ -77,6 +77,7 @@ tracing            = "0.1"
 tracing-subscriber = { version = "0.3", features = ["env-filter"] }
 anyhow             = "1"
 thiserror          = "1"
+async-trait        = "0.1"
 
 # Dev / bench
 criterion = "0.5"