Add 14 public methods (xdg_bin_home_or, xdg_cache_home_or, etc.) that check the corresponding XDG environment variable and fall back to a caller-provided default instead of platform-specific defaults. Each method is documented with examples and tested for three cases: returns XDG value when set, returns default when unset, and returns default when XDG contains a relative path. Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>