DH Ratchet Step

When a party sends a message after receiving one from the peer with a new DH public key, they perform a DH ratchet step:

Where:

The flow:

1. Alice sends msg1 (DH pub A1).
2. Bob receives msg1 → DH ratchet receive (uses A1 + Bob's current DH B0). Now has shared state.
3. Bob sends msg2 (DH pub B1).
4. Alice receives msg2 → DH ratchet receive with B1.
5. Alice sends msg3 (DH pub A2). Etc.

Ratchet advances only when direction CHANGES. If Alice sends 5 in a row, only one DH ratchet step total.

Headers per message:

{
    dh_pub: <Alice's current DH pub>
    pn: <previous chain length — for skipped key caching>
    n: <message number in current chain>
}

pn lets the receiver know "the previous chain had N messages; if you missed any, derive their mks now before switching chains."