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.

PARITY-CONTRACT.md

@@ -1,3 +1,12 @@
+> **RETIRED 2026-09.** The port it governed was completed and proven
+> (all golden vectors + both cross-directions passed), then superseded by
+> protocol v2 (`PROTOCOL-V2.md`) in 0.14.0: a red-team review found replay,
+> frame portability, and pre-auth allocation flaws baked into the v1 wire
+> format, unfixable behind parity. The v2 handshake rejects v1 traffic.
+> This file remains as the historical record of the port — what was
+> preserved, what was dropped, and the C++ byte formats the `Cereal` codec
+> still speaks for payloads.
+
 # OmniVox Port — Parity Contract
 
 Source: `omnivox/` (C++17/20, asio + cereal + CryptoPP + fmt)