Function install_with_loader 

pub async fn install_with_loader(
    mc_version: &str,
    instance_name: &str,
    base_dir: &Path,
    loader: &dyn LoaderInstaller,
    progress: ProgressFn,
) -> Result<()>

Full install: vanilla first, then overlay the injected loader.

instance_name (caller-chosen) names the subdirectory under instance/ and avoids same-version/different-loader collisions, e.g. "1.21.1-vanilla" vs "1.21.1-fabric".

install_with_loader("1.21.4", "1.21.4-fabric", &base_dir, &FabricInstaller::latest(), no_progress()).await?;

let loader = ForgeInstaller::new("/usr/lib/jvm/java-17/bin/java");
install_with_loader("1.20.1", "1.20.1-forge", &base_dir, &loader, no_progress()).await?;