docs(design): evaluate Chain-of-Responsibility for zero-copy path selection (#2133)

[oferchen]

Summary

• Inventories all eight zero-copy file-to-file primitives in fast_io (FICLONE, copy_file_range, io_uring batched copy, clonefile, fcopyfile, CopyFileExW, FSCTL_DUPLICATE_EXTENTS_TO_FILE, std::fs::copy) with file:line citations into crates/fast_io/src/platform_copy/dispatch.rs.
• Maps every decision point and fallback edge in the current cfg-cascade dispatch and audits which fall-through edges have test coverage.
• Sketches a ZeroCopyHandler trait + per-platform static chain variant and weighs pros (testability, telemetry hook, dedupe of cleanup) against cons (the dispatch is already 100 LoC, cfg-guards do not vanish, PlatformCopy is already a Strategy, the size-tier chain in copy_file_range.rs does not share its shape).
• Recommends keeping the cfg-cascade and capturing the only real wins (last-winner memoisation, cleanup helper extraction) with about 25 LoC of in-place edits rather than a trait refactor.
• Cross-references Restrict CI testing to Linux and fix packaging artifacts #1389 (ReFS reflink), Revise CI workflow for branded release builds #1414 / Split local copy error definitions into dedicated module #1749 (CopyFileExW), Optimize sparse zero detection #1932 (IOCP), docs: add initial cli parity matrix #2131 (FSCTL_SET_ZERO_DATA), Refine cross-platform packaging workflows #2130 (TransmitFile) and explains why each fits under the cascade rather than as a new chain member.

Test plan

• Doc-only change, no code touched.
• All file:line citations verified against crates/fast_io/src/platform_copy/dispatch.rs, types.rs, mod.rs, tests.rs, and crates/fast_io/src/copy_file_range.rs.
• No em-dashes; hyphens throughout.