docs(design): async SSH pipe wrapper with read/write overlap (#1412)

[oferchen]

Summary

• Adds docs/design/async-ssh-pipe-wrapper.md capturing the wrapper-level design for swapping the SSH subprocess stdio onto tokio::process::Child stdio with read/write overlap.
• Compares tokio::process::ChildStdin/ChildStdout against a raw AsyncFd construction over the existing pipes and recommends the former.
• Names what async actually adds over today's thread-per-half model (thread elimination for fan-out clients, collapsed stderr drain and connect watchdog, event-loop scheduling), and what it does not add (no fork/exec reduction, no io_uring socket fast path, no wire change).
• Records a sync-vs-async bench matrix anchored on the 1 MB/s emulated link from Improve cross-compilation tooling autodetection #1889, plus LAN, slow-disk, and fan-out rows.
• Recommendation: defer implementation until Normalize quoted fallback keywords #1935 (async daemon listener) lands so tokio runtime ownership is settled in one place before adding a second async I/O surface.

Test plan

• Pure docs change; no code touched.
• cargo fmt --all ran clean (no formatting changes).
• No em-dashes, no forbidden references in the doc body.
• Cross-references against fix(ssh): resolve goodbye-phase deadlock + load ~/.ssh/config in russh path #4154 (SSH perf fix), docs(design): async migration plan with incremental adoption strategy (#1594) #4186 (async migration plan), docs(audits): SSH socketpair vs pipe transport feasibility (#1687) #4193 (single-socketpair NACK audit), docs(design): async runtime evaluation for SSH transport (#1411) #4194 (async SSH evaluation), Improve cross-compilation tooling autodetection #1889 (sync-vs-async bench), Normalize quoted fallback keywords #1935 (async daemon), Add canonical rsync entrypoints #1795-Handle forced missing cargo tools in packaging tests #1806 (SshTransport pieces), and the underlying audits cited inline with file:line.