Function is_fallback_active

pub fn is_fallback_active() -> bool

true when the active theme was produced by the embedded B/W fallback because ThemeDocument::find("default") failed at first-access. Flipped back to false the moment a consumer calls set_active_document with any document (even another call to the same fallback — the point is “this state came from an explicit install, not from the missing-theme code path”).

The draw layer reads this to decide whether to stamp each surface with the red “install ltk-theme-default” banner; apps can read it too, e.g. to log a diagnostic or surface a first-run installation helper.