test: cover Windows encoded roots

[hyfdev]

Summary

• cover all six Windows root-identifier comparison slots that can contain arbitrary native encoding: UNC server/share, verbatim UNC server/share, DeviceNS, and generic verbatim names
• prove public relative APIs fold ASCII case only, distinguish lone surrogate units that have the same lossy rendering, do not apply Unicode case folding, and keep DeviceNS separate from generic verbatim names
• verify clean and dirty absolutization preserves encoded root identifiers through strict, fallible, and explicit-cwd entry points, including ordinary and root-relative receivers with borrowed and owned cwd values

The matrix first verifies the expected std::path::Prefix variant so a malformed fixture cannot silently exercise an ordinary component. Drive and verbatim-drive identifiers are intentionally excluded because Rust recognizes only ASCII alphabetic drive letters; a non-ASCII or surrogate pseudo-drive would not enter the drive-prefix comparator.

Same-root invalid-encoding results assert exact wide output without requiring Cow::Borrowed: non-UTF performance is not an optimization gate. Different-root results remain required to return the documented owned normalized target.

This PR adds one Windows-only integration test file. It does not change production code, dependencies, benchmarks, or benchmark inputs.

Validation

• SUGAR_PATH_EXPECT_CACHED_CURRENT_DIR=0 cargo +1.97.0 test --locked -p sugar_path
• SUGAR_PATH_EXPECT_CACHED_CURRENT_DIR=1 cargo +1.97.0 test --locked --workspace --all-features
• cargo +1.97.0 fmt --all -- --check
• cargo +1.97.0 clippy --locked --workspace --all-targets --all-features -- -D warnings
• production library check and Clippy for x86_64-pc-windows-msvc
• the changed integration test checked and linted for MSVC through a dependency-only harness, avoiding the repository dev-only mimalloc build that requires a local Windows SDK
• two independent adversarial reviewers found an over-constrained non-UTF Cow assertion, a missing encoded-cwd root-relative branch, and a missing DeviceNS/verbatim isolation pair; all three are fixed

At the rebased head 31ed4cc, all eight repository Actions jobs and external CodSpeed Analysis pass. Native x86_64-pc-windows-msvc executes both encoded-root tests successfully under default and all features.

[codspeed-hq]

Merging this PR will not alter performance

✅ 228 untouched benchmarks
⏩ 38 skipped benchmarks¹

---

_{Comparing agent/windows-encoded-root-matrix (31ed4cc) with main (36ea1df)}

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. ↩

[Copilot]

Copilot was unable to review this pull request because the user who requested the review has reached their quota limit.