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-tunnel/Cargo.toml

@@ -6,9 +6,9 @@ license.workspace = true
 description = "Aura tunnel: cross-platform TUN, split-tunnel routing, DNS"
 
 [dependencies]
-aura-transport.workspace = true
 aura-proto.workspace = true
 aura-crypto.workspace = true
+async-trait.workspace = true
 tokio.workspace = true
 bytes.workspace = true
 ipnetwork.workspace = true