feat(rsync_io): ChannelReader/Writer AsyncRead/AsyncWrite adapters (#1797)

[oferchen]

Summary

• Adds ChannelReader and ChannelWriter in a new crates/rsync_io/src/channel_adapter.rs module that bridge tokio::sync::mpsc::{Receiver, Sender}<Vec<u8>> to tokio::io::AsyncRead / AsyncWrite.
• Adds a new async-ssh feature on rsync_io. The module is fully gated behind this feature, so default builds are unchanged.
• Exposes pair(capacity) returning two cross-connected (ChannelReader, ChannelWriter) halves for in-memory duplex streams.

Why

Companion to the in-flight async SSH transport (task #1796). The adapters give the async transport a way to expose its stdio over an in-process duplex channel while still satisfying APIs that require raw async stream traits.

Implementation notes

• ChannelReader drains a one-time buffered: Vec<u8> slice before pulling the next chunk from the channel, so oversized chunks are served across multiple poll_read calls without dropping bytes.
• ChannelWriter first tries try_send, then falls back to reserve_owned(). The reservation future is stored on the writer so the channel's wait-queue registration survives across poll_write calls and the task is woken when capacity frees up.
• poll_flush is a no-op (the channel itself provides ordering). poll_shutdown drops the sender, surfacing EOF on the read side.
• A closed receiver surfaces on the writer as io::ErrorKind::BrokenPipe.
• Uses Vec<u8> rather than bytes::Bytes since bytes is not a workspace dependency.

Test plan

• cargo fmt --all clean
• CI fmt + clippy
• CI nextest (stable) - covers the new unit tests when the async-ssh feature is enabled
• CI Windows / macOS / Linux musl (pure tokio, cross-platform)

Unit tests (gated cfg(test) under the async-ssh feature)

• round_trip_bytes - write then read N bytes.
• oversized_chunk_is_drained_across_reads - reader serves an over-large chunk across multiple poll_read calls.
• backpressure_blocks_until_reader_drains - bounded capacity 1 forces the second write to suspend until the reader pulls the first message.
• dropping_writer_yields_eof - dropping the writer gives reader Ok(0).
• shutdown_yields_eof - explicit shutdown() after a write still delivers buffered data, then EOF.
• write_after_shutdown_is_broken_pipe - subsequent writes after shutdown return BrokenPipe.
• write_to_closed_reader_is_broken_pipe - dropping the receiver surfaces BrokenPipe to the writer.
• duplex_pair_exchanges_data_both_ways - pair() gives a working bidirectional in-memory stream.