test: cover Windows invalid drive paths

[hyfdev]

Summary

• cover exact invalid-wide component comparison in Windows drive-relative paths
• verify equal invalid components cancel while distinct components with the same lossy rendering remain distinct
• preserve invalid-wide components when a drive-relative path is resolved against borrowed or owned explicit cwd

Why

The existing Windows invalid-encoding matrix covered unprefixed lexical, absolute, and root-relative paths, but not drive-relative helpers.

Three controlled errors survived the existing suite: comparing drive-relative components through lossy Unicode, refusing to compare any invalid-wide components, and dropping an invalid component while resolving a drive-relative path. The new raw UTF-16 assertions distinguish all three cases through the public APIs.

Validation

• direct rustc --target x86_64-pc-windows-msvc --test ... --emit=metadata -D warnings cross-compilation against the built Windows library
• default and cached invalid-encoding suites on the local Unix host
• cargo fmt --all --check and git diff --check
• two adversarial review rounds; the first found the missing equal-invalid control row, and the final round passed after it was added

Native Windows execution is provided by the existing GitHub Actions Windows job.

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