Session 46: Noise IK peer authentication implementation

Implement Noise Protocol Framework for peer authentication using the IK
pattern, which allows responders to learn initiator identity from the
encrypted handshake message.

Protocol: Noise_IK_secp256k1_ChaChaPoly_SHA256
- Message 1: 82 bytes (ephemeral + encrypted static)
- Message 2: 33 bytes (ephemeral only)

New noise.rs module (~600 lines):
- HandshakeState: Manages handshake for both initiator/responder roles
- NoiseSession: Post-handshake symmetric encryption
- CipherState: ChaCha20-Poly1305 with nonce counter
- SymmetricState: HKDF-SHA256 key derivation

PeerConnection integration:
- Simplified HandshakeState enum (Initial/SentMsg1/ReceivedMsg1/Complete/Failed)
- start_handshake(): Initiator generates msg1
- receive_handshake_init(): Responder processes msg1, discovers identity
- complete_handshake(): Initiator completes with msg2
- take_session(): Extract NoiseSession for ActivePeer

Identity helpers:
- Identity::keypair() for Noise operations
- PeerIdentity::from_pubkey_full() preserves parity for ECDH
- PeerIdentity::pubkey_full() returns full key or derives with even parity

Node changes:
- initiate_peer_connection() now starts handshake and sends msg1
- Method is async to use transport's async send

All 219 tests pass.

Cargo.toml

@@ -6,6 +6,8 @@ edition = "2024"
 [dependencies]
 secp256k1 = { version = "0.30", features = ["rand", "global-context"] }
 sha2 = "0.10"
+hkdf = "0.12"
+chacha20poly1305 = "0.10"
 rand = "0.8"
 thiserror = "2.0"
 bech32 = "0.11"