Function load_with_fallback 

pub(crate) async fn load_with_fallback<F: Fn() -> bool>(
    cache: &ThumbnailCache,
    asset_id: &str,
    requested: ThumbnailSize,
    is_cancelled: &F,
) -> Result<Texture, String>

Try requested bucket, then walk fallback_bucket chain on 404. Only the 404 path is retried — auth, network, etc. surface unchanged.