fix: restore platform-specific path separators for glob v13 compatibility

- Revert should_normalize_backslashes to original behavior:
  - On Windows with posix: false, output backslashes (matching glob)
  - On Windows with posix: true, output forward slashes
  - On Unix, always output forward slashes

- Update Rust tests with p() helper for platform-appropriate paths
- Update Node.js tests with normalizePaths() for cross-platform comparison
- Add platformPath() and platformPaths() helpers to test harness

Glob v13 uses backslashes on Windows by default. This restores
that behavior so globlin matches glob exactly on all platforms.

Author: richiemcilroy