feat: wire MacosWriter into the disk-commit dispatcher

[oferchen]

Summary

• Adds a Writer::Macos(MacosWriter) variant in crates/transfer/src/disk_commit/writer.rs, guarded by #[cfg(target_os = "macos")].
• Extends make_writer in crates/transfer/src/disk_commit/process.rs with a macOS arm that wraps the per-file File in fast_io::MacosWriter::from_file(file, target_size) when !use_sparse && append_offset == 0, mirroring the existing io_uring and IOCP arms.
• Adds MacosWriter::sync() so the dispatcher's flush_and_sync(do_fsync, ...) can drain pending writev buffers and fsync the underlying file.

Motivation

MacosWriter (crates/fast_io/src/macos_io.rs) has shipped for a while: it pairs F_NOCACHE (above the 1 MiB threshold) with writev(2) flushes (up to 64 iovecs). It is re-exported from crates/fast_io/src/lib.rs, but the disk-commit thread still constructs ReusableBufWriter directly on macOS, so the optimization has zero callers in the workspace.

The audit at docs/audits/macos-fastio-fallback.md (PR #4008, task #1652) called out this gap and recommended exactly the wiring in this PR as the fix for #1657.

Behaviour

• macOS, non-sparse, append_offset == 0: routes through Writer::Macos(MacosWriter). Files >= 1 MiB get F_NOCACHE (page-cache bypass for tree-wide transfers larger than RAM); writes >= 256 KiB flush via writev covering up to 64 iovecs per syscall.
• macOS, sparse or append_offset != 0: falls through to Writer::Buffered(ReusableBufWriter). MacosWriter does not implement Seek and would not preserve the file position used by sparse/append mode.
• Other platforms: unchanged. The macOS arm is gated; Writer::Macos does not exist off-target.

Test plan

• CI fmt + clippy across all platforms.
• cargo nextest run -p transfer on macOS picks up make_writer_selects_macos_for_non_sparse_zero_offset and make_writer_falls_back_to_buffered_when_seek_required.
• Linux/Windows builds remain unchanged - the macOS arm is #[cfg(target_os = "macos")] and the dispatcher signature only adds a size_hint: u64 parameter.

References

• Audit: docs/audits/macos-fastio-fallback.md (PR docs: audit macOS fast_io fallback vs Linux io_uring (#1652) #4008).
• Task: Fix IPv6 zone identifier parsing in module list requests #1657 - wire F_NOCACHE + writev into disk-commit.
• Closes Enforce coverage thresholds in CI #1652 cleanly per the audit's section 4.1 recommendation.