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.pub

@@ -0,0 +1 @@
+ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAICtBzWJ6iltFPtzzRq7fxqJ4MdXrukOCk5YEK293DYjl markbase_ssh_host_key