feat(paths): add fromUnixPath for MSYS-to-Windows path conversion

[John-David Dalton (jdalton)]

Summary

• Adds fromUnixPath() as the inverse of toUnixPath() — converts MSYS/Git Bash paths (/c/path/to/file) back to native Windows format (C:/path/to/file)
• Needed for sfw shim creation on Windows runners where command -v returns MSYS-style paths that PowerShell and cmd.exe can't resolve
• On Unix platforms, passes through normalization unchanged (same behavior as toUnixPath)

Test plan

• 14 new test cases covering drive letters, edge cases, Buffer/URL input, round-trip with toUnixPath
• All 101 normalize tests pass
• Build passes
• Lint and TypeScript check pass