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-pki/Cargo.toml

@@ -0,0 +1,15 @@
+[package]
+name = "aura-pki"
+version.workspace = true
+edition.workspace = true
+license.workspace = true
+description = "Aura PKI: CA, X.509 issuance and mutual-auth verification"
+
+[dependencies]
+rcgen.workspace = true
+rustls.workspace = true
+rustls-pki-types.workspace = true
+x509-parser.workspace = true
+uuid.workspace = true
+thiserror.workspace = true
+anyhow.workspace = true