docs(design): macOS F_NOCACHE and writev fallback (#1657)

[oferchen]

Summary

• Design note for the near-term macOS-optimized fallback that pairs F_NOCACHE on the destination fd with writev(2) for the buffered-write path. Targets the gap between today's plain BufWriter<File> on macOS and the longer-term kqueue backend in Adjust CI coverage gate and extend cross-platform builds #1385.
• Composes with the kqueue design (docs/design/macos-kqueue-fast-io.md Section 7.2) and fills in the WritevAsyncWriter branch named in the AsyncFileWriter audit (Refactor module list client into modular structure #1655).
• Includes API surface change in fast_io (WritevWriter, WritevPolicy, --writev-macos flag), interaction with sparse mode (falls through to Buffered for Seek), comparison vs full kqueue (Adjust CI coverage gate and extend cross-platform builds #1385) and dispatch_io (Fix client module test visibility and import duplication #1653, completed), test plan against the existing macOS CI matrix, and open questions covering rename targets, threshold tuning, --inplace, SMB, fcopyfile, and AppleDouble (Align Windows cross-compile matrix with documentation #1907).

Test plan

• CI fmt/lint passes on the docs-only change.
• Verify the document renders cleanly on GitHub.
• Confirm cited file:LINE references resolve against current master.