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

Cargo.lock

@@ -5436,6 +5436,7 @@ dependencies = [
  "bytes",
  "cap-std",
  "cmac 0.7.2",
+ "ctr 0.9.2",
  "getrandom 0.4.2",
  "hex",
  "hmac 0.12.1",