perf: avoid redundant relative scans on macOS ARM64

[hyfdev]

Summary

• Scan the shared component prefix before normalization classification for absolute relative calls on macOS AArch64 with NEON.
• Validate only unmatched suffixes when an upward result is already necessarily owned, while retaining full-input validation before borrowing descendants.
• Record Windows, macOS, and Linux as the performance targets; other platforms remain correctness-supported but are not performance goals.
• Add exhaustive short-path result and Cow parity coverage plus multibyte UTF-8 prefix and mismatch cases.

Scope

The optimized implementation is compiled only for macOS AArch64 with NEON. macOS x86_64, Windows, Linux, and other operating systems compile the previous implementation unchanged. This PR adds no unsafe and no new benchmark cases.

Performance

Compared with accepted baseline commit 886fc357f228e8a89e49103fdaa88db7eab70877 on an Apple M3 Pro using the existing relative benchmark, with freshly built binaries in isolated target directories, 150 samples, a 2-second warm-up, and a 5-second measurement:

Existing case	Natural result	PathBuf result
short_common_prefix	9.41% faster	9.37% faster
deep_siblings	68.31% faster	68.58% faster
different_subtrees	27.83% faster	29.84% faster
module_to_cwd	no change	within 1% noise
same_directory	within noise	3.17% faster

Two full dot_slow_path comparisons moved in opposite directions: one owned-result run regressed by 2.73%, then the repeat improved by 1.94%. A balanced eight-run alternation did not reproduce a regression, so this row is treated as having no demonstrated change rather than as a gain.

AArch64 release IR grows from 16,665 lines and 430 copies to 16,777 lines and 431 copies. macOS x86_64 remains exactly 15,469 lines and 385 copies.

Validation

• cargo fmt --all -- --check
• cargo test --all-features
• cargo clippy --all-targets --all-features -- -D warnings
• RUSTDOCFLAGS="-D warnings" cargo doc --no-deps --all-features
• cargo check --lib --target x86_64-pc-windows-gnu
• cargo check --lib --target x86_64-pc-windows-msvc
• cargo check --lib --target x86_64-unknown-linux-gnu

An independent adversarial review found no remaining correctness, ownership, UTF-8, platform-scope, or safety blocker.

[codspeed-hq]

Merging this PR will not alter performance

✅ 228 untouched benchmarks
⏩ 38 skipped benchmarks¹

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_{Comparing agent/relative-suffix-scan-20260714 (bfc7218) with main (886fc35)}

Footnotes

1. 38 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

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