Function toRelativePath

Resolve filePath relative to rootPath and return a clean unix-style relative path, or null if the file is outside the root.

• Handles both absolute and relative input paths.
• Resolves . and .. segments via path.resolve.
• Converts backslashes to forward slashes on Windows.
• On POSIX, backslashes are preserved (they are valid in filenames).
• Strips any leading ./.
• Returns null for paths that resolve outside rootPath.
• Returns null for empty strings.