docs(fast_io): design AsyncFileWriter trait (#1655)

[oferchen]

Summary

• Design audit for a platform-agnostic AsyncFileWriter trait that hides Linux io_uring, Windows IOCP, and macOS dispatch_io / writev behind a single async-friendly surface in fast_io.
• Cites every existing call site that branches on cfg(target_os) or feature flags today (disk-commit Writer enum, make_writer, transfer_ops/response.rs:108, generator basis-read path, lib.rs module gates) and shows how each collapses to a single trait method.
• Recommends a four-phase landing sequence: lift IOCP first, wrap io_uring (including IoUringDiskBatch), add macOS impl per Fix IPv6 zone identifier parsing in module list requests #1657, then retire IoUringOrStdWriter / IocpOrStdWriter from engine and transfer.

Closes #1655.

Verification

Docs-only change. New file: docs/audits/async-file-writer-trait.md. No .rs, Cargo.toml, or build-config changes. No code paths exercised. Cross-references existing audits #1653 (docs/audits/macos-dispatch-io.md), #1654 (docs/audits/bsd-aio.md), #1086 (docs/audits/disk-commit-iouring-batching.md), #1899 (docs/audits/windows-iocp-benchmark.md).