feat(singbox-aura,tools): Go port of Aura UDP client + KAT bridge to Rust

Lays the foundation for sing-box mobile clients (Option B from
docs/sing-box.md): an independent Go module that speaks the AuraVPN wire
protocol byte-for-byte. Proof of equivalence is in KAT tests cross-loaded
from a Rust-side deterministic vector exporter.

- tools/export-kat (new Rust bin in workspace): captures a handshake +
  derived keys + a sealed datagram record + a knock token using seeded
  RNGs (rand::rngs::StdRng + ml-kem's *_deterministic public API), emits
  JSON. Reproducible byte-for-byte.
- singbox-aura/ (new Go module, ~3000 LOC, 22 files):
  - aura/frame: 5-byte protocol header + Frame{Data,Ping,Pong,Close,
    Control} + magic envelope (0xAA,0xAA,0xC0,0x01) — encode/decode
    matching aura-proto::frame.
  - aura/crypto: hybrid X25519 + ML-KEM-768 (stdlib crypto/ecdh +
    crypto/mlkem on Go 1.24+; falls back to circl on older Go via a
    documented swap), HKDF-SHA256 derive_session_keys, ChaCha20-Poly1305
    with the **LE(u64 counter) || [0;4]** nonce scheme that matches
    aura-crypto::AeadKey/AeadSession.
  - aura/handshake: client_handshake state machine reproducing protocol.md
    §6.2 exactly (CH→SH→ServerAuth→ClientAuth→Finished×2; transcript hash;
    ECDSA-P256 transcript signature; HMAC-SHA256 Finished).
  - aura/session: DatagramSender/Receiver + 64-wide sliding replay window.
  - aura/transport: reliable HS-adapter (DTLS-flight retransmit) + UDP
    datagram data path + 16-byte HMAC port-knock with ±1-minute window.
  - aura/outbound: sing-box-shaped shim (interface signatures only — sing-
    box upstream registration is one more step, documented in README).
  - cmd/aura-client: standalone Go binary; reads client.toml via
    pelletier/go-toml/v2 and connects to a real aura server. Validates
    end-to-end interop with the Rust side.
- KAT: 6 comparisons against Rust vectors — session_keys (HKDF), hybrid
  KEM ek/encaps roundtrip, c2s + s2c Finished HMAC, sealed datagram
  record at seq=2 (incl. 16-byte Poly1305 tag), knock token. All byte-
  for-byte.

Go: 29 tests across 5 packages, all green. Only deps: golang.org/x/crypto
and pelletier/go-toml/v2. Rust: 293 tests still green; tools/export-kat
added to workspace members.

v1 limits documented in singbox-aura/README.md: UDP-only (no TCP/QUIC
fallback yet), no cell padding / cover traffic, no relay/exit role, no
multi-hop, sing-box upstream-registration sketch (vendor sagernet/sing-box +
init() RegisterOutbound) for follow-up.

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

singbox-aura/aura/crypto/kdf.go

@@ -0,0 +1,46 @@
+package crypto
+
+import (
+	"crypto/sha256"
+	"hash"
+
+	"golang.org/x/crypto/hkdf"
+)
+
+// HKDFInfo is the domain-separation string bound into the HKDF expansion.
+// MUST match HKDF_INFO in crates/aura-crypto/src/kdf.rs.
+var HKDFInfo = []byte("aura-v1-session")
+
+// SessionKeyLen is the size of one directional AEAD key.
+const SessionKeyLen = 32
+
+// SessionKeys is the pair of directional 256-bit keys produced by the HKDF expansion.
+type SessionKeys struct {
+	ClientToServer [SessionKeyLen]byte
+	ServerToClient [SessionKeyLen]byte
+}
+
+// DeriveSessionKeys runs HKDF-SHA256 with
+//
+//	salt = client_nonce || server_nonce (64 bytes)
+//	IKM  = x25519_ss || mlkem_ss        (64 bytes)
+//	info = "aura-v1-session", OKM 64 bytes -> (c2s, s2c)
+//
+// matching the production helper in crates/aura-crypto/src/kdf.rs byte-for-byte.
+func DeriveSessionKeys(shared *HybridSharedSecret, clientNonce, serverNonce [32]byte) *SessionKeys {
+	salt := make([]byte, 64)
+	copy(salt[:32], clientNonce[:])
+	copy(salt[32:], serverNonce[:])
+
+	ikm := shared.Concat()
+	hk := hkdf.New(func() hash.Hash { return sha256.New() }, ikm, salt, HKDFInfo)
+	okm := make([]byte, 64)
+	if _, err := hk.Read(okm); err != nil {
+		// HKDF-Read for 64 bytes from SHA-256 is infallible; treat any error as a bug.
+		panic(err)
+	}
+	var keys SessionKeys
+	copy(keys.ClientToServer[:], okm[:32])
+	copy(keys.ServerToClient[:], okm[32:])
+	return &keys
+}