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-crypto/Cargo.toml

@@ -0,0 +1,23 @@
+[package]
+name = "aura-crypto"
+version.workspace = true
+edition.workspace = true
+license.workspace = true
+description = "Aura cryptographic core: hybrid X25519 + ML-KEM-768 KEM, HKDF, ChaCha20-Poly1305"
+
+[dependencies]
+ml-kem.workspace = true
+x25519-dalek.workspace = true
+hkdf.workspace = true
+hmac.workspace = true
+sha2.workspace = true
+chacha20poly1305.workspace = true
+rand.workspace = true
+rand_core.workspace = true
+zeroize.workspace = true
+subtle.workspace = true
+thiserror.workspace = true
+
+[dev-dependencies]
+hex.workspace = true
+criterion.workspace = true

crates/aura-crypto/src/lib.rs

@@ -0,0 +1 @@
+//! aura-crypto — cryptographic core (skeleton; implemented in Wave 1).