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/outbound/README.md

@@ -0,0 +1,37 @@
+# Aura outbound for sing-box
+
+`outbound.Outbound` exposes a sing-box-shaped surface (`Network() / DialContext / ListenPacket`)
+without importing `github.com/sagernet/sing-box`. This keeps the build self-contained for v1;
+the next step is to vendor the sing-box module, register Aura via `init()` and add the JSON
+options struct.
+
+## Integration sketch (Option B from `docs/sing-box.md`)
+
+```go
+import (
+    "github.com/sagernet/sing-box"
+    "github.com/sagernet/sing-box/adapter"
+    "github.com/sagernet/sing-box/option"
+    auraout "github.com/aura/singbox-aura/aura/outbound"
+)
+
+func init() {
+    sing-box.RegisterOutbound(auraout.Tag, func(ctx context.Context, router adapter.Router, logger logger.ContextLogger, tag string, options option.Outbound) (adapter.Outbound, error) {
+        // Translate option fields to handshake.ClientConfig + transport.Options.
+        // Construct &auraout.Outbound{...} and adapt to adapter.Outbound (DialContext signature).
+    })
+}
+```
+
+The exact `option.Outbound` schema is up to you — at minimum it needs:
+
+* `server` (host:port)
+* `tls.ca_cert_path` (PEM)
+* `tls.cert_path`, `tls.key_path` (PEM, ECDSA P-256)
+* `tls.server_name` (DNS SAN to verify in the server leaf)
+* optional `knock_enabled`, `knock_secret_source = "ca_fingerprint"`
+
+The packet path is **opaque IP** — Aura tunnels inner IP packets exactly as the existing Rust
+client does. The router writes IPv4/IPv6 packets to the returned `net.PacketConn`; the same
+conn yields incoming packets on `ReadFrom`. Multi-flow demultiplexing is the router's job, not
+ours.