Add EC-SRP5 authentication (RouterOS >= 6.43)

Client: auto-detects 03 response and performs EC-SRP5 handshake
Server: --ecsrp5 flag enables Curve25519 Weierstrass EC-SRP5 auth
  btest -s -a admin -p password --ecsrp5

Protocol: [len][payload] framing (no 0x06 handler, unlike Winbox)
Crypto: Curve25519 in Weierstrass form, SHA256, SRP key exchange

Based on MarginResearch/mikrotik_authentication (Apache 2.0).
Verified against MikroTik RouterOS 7.x via MITM protocol analysis.

34 tests (10 unit, 6 EC-SRP5 integration, 8 base integration, 10 doc-tests).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/protocol.rs

@@ -188,6 +188,7 @@ pub async fn send_command<W: AsyncWriteExt + Unpin>(
     Ok(())
 }
 
+#[allow(dead_code)]
 pub async fn recv_command<R: AsyncReadExt + Unpin>(reader: &mut R) -> Result<Command> {
     let mut buf = [0u8; 16];
     reader.read_exact(&mut buf).await?;