qol: implement easier building and distribution

include/kappa/build/build.hpp

@@ -0,0 +1,23 @@
+#pragma once
+
+#include "kappa/resolve/plan.hpp"
+
+#include <string>
+
+namespace kappa::build {
+
+struct BuildResult {
+    bool ok = false;
+    std::string phase;  // phase that failed, e.g. "prepare" or "build"
+    std::string error;
+};
+
+// Build a single package step into work_dir.
+//
+// NOTE: compile-only stub — the real build backend was never committed.
+// Always fails so the rebuild pipeline never records a fake install.
+BuildResult build(const resolve::BuildStep& step,
+                  const std::string& work_dir,
+                  int jobs);
+
+} // namespace kappa::build