chore: scaffold Aura workspace skeleton (Stage 0)

- 6-crate Cargo workspace, dependency tree frozen (cargo check green in ~1m)
- ml-kem 0.3 (FIPS 203) replaces spec's pqcrypto-kyber for ML-KEM-768
- fix invalid target-gated workspace.dependencies: Windows deps (wintun/windows)
  declared untargeted, cfg-gated per-crate in aura-tunnel
- version bumps vs spec: tun 0.8, rcgen 0.14, wintun 0.5
- stub lib/main per crate; real implementations land wave by wave

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

crates/aura-tunnel/Cargo.toml

@@ -0,0 +1,25 @@
+[package]
+name = "aura-tunnel"
+version.workspace = true
+edition.workspace = true
+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
+tokio.workspace = true
+bytes.workspace = true
+ipnetwork.workspace = true
+hickory-resolver.workspace = true
+tracing.workspace = true
+thiserror.workspace = true
+anyhow.workspace = true
+
+[target.'cfg(not(windows))'.dependencies]
+tun.workspace = true
+
+[target.'cfg(windows)'.dependencies]
+wintun.workspace = true
+windows.workspace = true

crates/aura-tunnel/src/lib.rs

@@ -0,0 +1 @@
+//! aura-tunnel — TUN interface and split tunneling (skeleton; implemented in Wave 3).