Function execute_build_pipeline 

pub fn execute_build_pipeline(
    plugins: &PluginManager,
    ctx: &PluginContext,
    build_dir: &Path,
    content_dir: &Path,
    site_dir: &Path,
    template_dir: &Path,
    quiet: bool,
) -> Result<()>

Runs the build half of the pipeline: before_compile → compile → after_compile. Does not start the dev server.

Extracted from run() so the actual build can be unit-tested against a tempdir without booting an HTTP server.