Phase 1.1: SMB3 encryption module (AES-CTR + HMAC)

- Add encryption.rs with Smb3Encryption struct
- Implement AES-128-CTR + HMAC-SHA256 (simplified approach)
- Add TransformHeader struct for SMB2 TRANSFORM_HEADER
- 3 unit tests pass (encrypt/decrypt roundtrip + signature verification)
- Total: ~180 lines of code

config/ssh_host_keys/ssh_host_ed25519_key.meta

@@ -0,0 +1,6 @@
+{
+  "created_at": 1782062629,
+  "expires_at": 1813598629,
+  "fingerprint": "YhvUXPPA1xlmnfJ9H0axfLsV5wve9QMiRQ2eFarT/D4=",
+  "key_type": "ed25519"
+}