0.15.0: simple builder API, v2.1 framing, stream lane, benches

- omnivox::server::<T>()/client::<T>() builders; plain closures, enum in,
  match in the callback — the original C++ UX with the ceremony removed
- serde + bincode replace the hand-rolled Cereal codec (vectors.rs still
  governs the crypto leaves via the C++ golden oracles)
- v2.1 frames: u32 len | type | ct | tag; nonce derived from the sequence
  counter (no wire nonce), type byte authenticated as AAD; any frame-level
  failure is fatal, authenticated-but-undecodable payloads are skipped
- stream lane: send_stream/on_stream, 1 MiB frames, shared sequence with
  messages (exact sender interleaving preserved); see dump-agent/viewer
- per-connection bounded queues + writer tasks; reply/reply_stream are
  non-blocking try_send lanes for callbacks
- benches: seal/open criterion group + transport harness (spray msg/s,
  stream GiB/s, raw loopback baseline)
- profile.dev.package scrypt/pbkdf2 opt-level 3: golden-vector scrypt at
  N=2^17 was unusable in debug builds
- PROTOCOL-V2.md and README rewritten for v2.1

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "omnivox"
-version = "0.14.0"
+version = "0.15.0"
 edition = "2024"
 license = "GPL-3.0-or-later"
 description = "Encrypted async TCP client-server messaging: X25519 handshake, ChaCha20-Poly1305, replay-proof sequencing (OmniVox protocol v2)"
@@ -13,10 +13,28 @@ rand_core = { version = "0.6", features = ["getrandom"] }
 sha2 = "0.10"
 x25519-dalek = { version = "2", features = ["zeroize"] }
 scrypt = "0.11"
+serde = { version = "1", features = ["derive"] }
+bincode = { version = "2", features = ["serde"] }
 thiserror = "2"
 tokio = { version = "1", features = ["rt-multi-thread", "net", "io-util", "time", "sync", "macros"] }
 tracing = "0.1"
 
 [dev-dependencies]
+serde = { version = "1", features = ["derive"] }
+bincode = { version = "2", features = ["serde"] }
 tracing-subscriber = { version = "0.3", features = ["fmt"] }
-tokio = { version = "1", features = ["full"] }
+tokio = { version = "1", features = ["full"] }
+criterion = { version = "0.5", features = ["async_tokio"] }
+
+[[bench]]
+name = "transport"
+harness = false
+# scrypt at N=2^17 (golden vectors) is unusable in unoptimized builds.
+[profile.dev.package.scrypt]
+opt-level = 3
+[profile.dev.package.pbkdf2]
+opt-level = 3
+[profile.dev.package.salsa20]
+opt-level = 3
+[profile.dev.package.sha2]
+opt-level = 3