refactor: move PacketConnection trait to aura-proto; decouple tunnel from transport

Worktree isolation is unavailable in this environment, so make Wave 3 safe for
same-tree parallel work instead: the PacketConnection contract now lives in
aura-proto (stable) and aura-tunnel no longer depends on aura-transport. With
transport and tunnel both depending only on proto (and not each other), the two
crates are independent leaves and can be built/edited concurrently without one
breaking the other's build. proto: 13 tests still green.

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

crates/aura-proto/Cargo.toml

@@ -26,6 +26,8 @@ x509-parser.workspace = true
 # The handshake and session are async over tokio::io::{AsyncRead, AsyncWrite}, so tokio must be a
 # normal dependency (available via the workspace `full` feature set), not only a dev-dependency.
 tokio.workspace = true
+async-trait.workspace = true
+anyhow.workspace = true
 
 [dev-dependencies]
 tokio.workspace = true