Protocol v2: X25519 handshake, sequence-number nonces, hardened transport

Red-team review of the (byte-exact) v1 port found replay, cross-session
frame portability, pre-auth allocation amplification, and a symmetric
key that let either peer forge as itself — flaws inherent to the v1
wire format. v2 closes them:

- fixed 69-byte OVX2 hello; server validates before replying; v1 and
  wrong-version peers see an immediate close
- HKDF(ephemeral X25519 DH, PSK, transcript) -> direction-separated
  session keys; forward secrecy per connection
- strictly increasing u64 sequence nonces: any replay or reorder drops
  the connection (regression tests in tests/protocol_v2.rs)
- Config: message-size cap (64 KiB default), connection semaphore,
  idle/body-stall/handshake/send timeouts on both roles (client-side
  timers retire the old no-client-timeouts quirk)
- message processing inline per connection; user callbacks panic-
  isolated; poison-tolerant mutexes; demo key refused in release builds
- PROTOCOL-V2.md supersedes PARITY-CONTRACT.md (retired, kept as port
  history); Cereal payload encoding is unchanged

Payload serialization remains byte-exact with the original C++ (the
golden scrypt/cereal vectors still govern). Zero unsafe. Verified live:
server<->client handshake, encrypted echo, clean teardown.

Cuts over from the C++ implementation, which is deleted (phase 5 of the
port contract); it remains in the original gitea repository.

examples/omnivox-client.rs

@@ -5,7 +5,7 @@
 mod common;
 
 use common::{DatagramPayload, PASSWORD, SALT, SERVER_ADDR, SERVER_PORT};
-use omnivox::{ConnectionContext, ConnectionStatus, OmniVox, derive_encrypt_key};
+use omnivox::{Config, ConnectionContext, ConnectionStatus, OmniVox, derive_encrypt_key};
 use std::sync::Arc;
 
 #[tokio::main]
@@ -62,7 +62,13 @@ async fn main() -> Result<(), Box<dyn std::error::Error>> {
     });
 
     client
-        .connect_to_server(SERVER_ADDR, SERVER_PORT, Some(data_cb), connection_cb)
+        .connect_to_server(
+            SERVER_ADDR,
+            SERVER_PORT,
+            Config::default(),
+            Some(data_cb),
+            connection_cb,
+        )
         .await?;
 
     tokio::time::sleep(std::time::Duration::from_secs(15)).await;