test: cover Windows drive-relative resolution errors

[hyfdev]

Summary

• add a deterministic Windows-only regression test for embedded-NUL drive-relative paths
• require try_absolutize and both target/base roles of try_relative to preserve the native error kind and raw OS error, while the strict APIs panic
• require the test to register and execute in both default and cached_current_dir CI configurations, and record the contract in the testing strategy

Why

Windows drive-relative ambient resolution delegates to std::path::absolute. Existing coverage exercised successful invalid-wide resolution and Unix cwd failures, but not an error from this Windows-specific branch. A regression could therefore bypass the native per-drive resolution path or mishandle its error without being detected.

An embedded NUL is deterministic under the pinned Rust 1.97.0 standard library: conversion to the WinAPI input buffer returns InvalidInput before GetFullPathNameW, so the test does not depend on drive mappings, filesystem contents, or runner cwd state.

Impact

This is a test-only contract change. Production code, dependencies, benchmarks, and allocation baselines are unchanged.

Validation

• cargo fmt --all --check
• default and cached_current_dir workspace test suites
• cargo clippy --locked --workspace --all-targets --all-features -- -D warnings
• RUSTDOCFLAGS="-D warnings" cargo doc --locked -p sugar_path --all-features --no-deps
• MSVC and GNU Windows library plus test-metadata compilation in both feature configurations
• independent adversarial review with no findings
• PR Test run: all seven jobs passed, with the new test executing successfully on native Windows MSVC in both feature configurations

[hyfdev]

Adversarial review conclusion

Reviewed PR #71 (head 928118e5) as an adversarial pass over the test-only Windows drive-relative error coverage.

Verdict: approve — no open findings.

Scope checked

• .agents/docs/testing-strategy.md
• .github/workflows/test.yaml
• tests/windows_drive_relative_errors.rs
• surrounding production path for Windows drive-relative ambient resolution and try_* / strict API error contracts

Assessment

This fills a real branch gap: ambient drive-relative resolution goes through std::path::absolute, and the new integration test forces that path with an embedded NUL so failure does not depend on drive maps, filesystem state, or cwd. Against the pinned Rust 1.97.0 std, interior NULs are rejected with ErrorKind::InvalidInput before GetFullPathNameW, so the claimed determinism holds. Production try_absolutize / try_relative propagate via ?; strict APIs panic through .expect(...). CI registers the exact test name on the Windows host list for both default and cached_current_dir listing gates, and the testing-strategy note matches the wiring.

No correctness, flakiness, incomplete contract coverage, or documentation mismatches found.

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