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>

Cargo.lock

@@ -242,6 +242,8 @@ dependencies = [
 name = "aura-proto"
 version = "0.1.0"
 dependencies = [
+ "anyhow",
+ "async-trait",
  "aura-crypto",
  "aura-pki",
  "bincode",
@@ -281,9 +283,9 @@ name = "aura-tunnel"
 version = "0.1.0"
 dependencies = [
  "anyhow",
+ "async-trait",
  "aura-crypto",
  "aura-proto",
- "aura-transport",
  "bytes",
  "hickory-resolver",
  "ipnetwork",