Hybrid X25519 + ML-KEM-1024 handshake and full PQ integration

[disentangle-network]

Summary

Complete post-quantum integration across the full Nebula stack. Builds on #5 (PQ certificates).

Hybrid handshake (NIST SP 800-56C)

• X25519 DH for classical security (unchanged Noise IX state machine)
• ML-KEM-1024 KEM layered on top via handshake payload fields
• Both shared secrets mixed via HKDF-SHA256 into final symmetric keys
• Breaking either primitive alone is insufficient to compromise the session

Handshake flow for Curve_PQ

1. Initiator: generate ephemeral ML-KEM-1024 keypair, include KemPublicKey in NebulaHandshakeDetails
2. Responder: encapsulate to initiator's KEM pubkey, include KemCiphertext in response, mix into dKey/eKey
3. Initiator: decapsulate, mix into dKey/eKey
4. Tunnel established with hybrid symmetric keys

nebula-cert CLI

• nebula-cert ca -curve PQ generates ML-DSA-87 CA (V2 only)
• nebula-cert sign with PQ CA generates ML-KEM-1024 host certs
• nebula-cert keygen -curve PQ generates ML-KEM-1024 keypairs

PKI integration

• PQ certs marshal with full public key (cert pubkey is ML-KEM, not Noise PeerStatic)
• UnmarshalCertificateFromBytes for PQ cert reconstruction in handshake
• VerifyPrivateKey fixed for ML-KEM-1024 packed format

E2E test

• TestGoodHandshakePQ: full tunnel between two PQ nodes, bidirectional data transfer verified
• 0.41s handshake time
• Zero regressions in classical (X25519, P256) handshake tests

Security stack

Layer	Algorithm	Standard
Transport	AES-256-GCM	Already PQ-safe
Key exchange (classical)	X25519 DH	Defense-in-depth
Key exchange (PQ)	ML-KEM-1024	FIPS 203
Key combiner	HKDF-SHA256	NIST SP 800-56C
Certificate signatures	ML-DSA-87	FIPS 204

Test plan

• 7 KEM unit tests (keygen, encapsulate/decapsulate roundtrip, hybrid key mixing)
• TestGoodHandshakePQ e2e: PQ CA -> host certs -> handshake -> tunnel -> bidirectional data
• TestGoodHandshake (classical): zero regressions
• TestGoodHandshakeNoOverlap (V2 classical): zero regressions
• All 46 cert + noiseutil tests pass