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>

.gitignore

@@ -0,0 +1,13 @@
+/target
+**/*.rs.bk
+
+# Generated PKI material — never commit private keys/certs
+*.key
+*.crt
+*.pem
+/pki/
+/etc/aura/
+
+# Local editor/agent config
+.claude/settings.local.json
+.DS_Store