feat(secret-stream): add SecretStream::into_split() for owned read/write halves

[eshork]

Summary

Adds SecretStream::into_split() so a caller can own the read half and write half in separate tasks:

pub fn into_split(self) -> (
    SecretStreamReadHalf<ReadHalf<T>>,
    SecretStreamWriteHalf<WriteHalf<T>>,
)

It splits the underlying transport with tokio::io::split and moves the decryptor (Pull) into SecretStreamReadHalf and the encryptor (Push) into SecretStreamWriteHalf.

Motivation

A SecretStream is inherently bidirectional, but read() and write() both take &mut self, so full-duplex use over a single stream forces either an Arc<Mutex<...>> (the reader holds the lock across read_exact, starving writes) or a single-task select!(read, outbound) driver (dropping an in-flight read() mid-frame loses bytes -> framing desync -> AEAD failure -> link reset). Owned halves eliminate both hazards: because the two halves hold disjoint cipher state, neither task can cancel the other's in-flight framed read/write. This is exactly what a mux carrying many TCP flows over one SecretStream needs.

What changed

Purely additive to peeroxide-dht/src/secret_stream.rs:

• one import (ReadHalf, WriteHalf),
• SecretStream::into_split(),
• SecretStreamReadHalf<R> with read(),
• SecretStreamWriteHalf<W> with write() and shutdown(),
• two tests.

The half methods reuse the existing read_frame/write_frame helpers and the same Push/Pull state, so wire framing and AEAD behavior are byte-for-byte identical to the unsplit read()/write() (same 3-byte LE length prefix, same empty-keepalive skipping, same CiphertextTooShort guard). No change to the Noise handshake or the secretstream cipher.

Tests

cargo test -p peeroxide-dht passes, including two new tests: into_split_roundtrip (concurrent bidirectional bulk in separate tasks) and into_split_empty_keepalive (keepalive handling parity).