paths.build fix

internal/project/project_test.go

@@ -1113,6 +1113,52 @@ func TestValidateLayoutAllowsMissingAssetsDir(t *testing.T) {
 	}
 }
 
+// paths.build is an OUTPUT dir the builder creates (MkdirAll) before writing, so
+// a bare clone with no build dir yet must still validate/build with no pre-step
+// (R2/parity). Only a build path that exists but is not a directory is an error.
+func TestValidateLayoutAllowsMissingBuildDir(t *testing.T) {
+	root := t.TempDir()
+	mkdirAll(t, filepath.Join(root, "src"))
+	// deliberately do NOT create build/
+
+	proj := &Project{
+		Root: root,
+		Config: Config{
+			Module: ModuleConfig{Name: "Test", ResRef: "test"},
+			Paths:  PathConfig{Source: "src", Build: "build"},
+		},
+	}
+
+	if err := proj.ValidateLayout(); err != nil {
+		t.Fatalf("ValidateLayout returned error for missing build dir: %v", err)
+	}
+	if _, err := os.Stat(filepath.Join(root, "build")); !errors.Is(err, os.ErrNotExist) {
+		t.Fatalf("expected missing build dir to remain absent, got err=%v", err)
+	}
+}
+
+// A build path that exists but is a regular file is still a hard error.
+func TestValidateLayoutRejectsBuildDirThatIsAFile(t *testing.T) {
+	root := t.TempDir()
+	mkdirAll(t, filepath.Join(root, "src"))
+	if err := os.WriteFile(filepath.Join(root, "build"), []byte("i am a file, not a dir"), 0o644); err != nil {
+		t.Fatal(err)
+	}
+
+	proj := &Project{
+		Root: root,
+		Config: Config{
+			Module: ModuleConfig{Name: "Test", ResRef: "test"},
+			Paths:  PathConfig{Source: "src", Build: "build"},
+		},
+	}
+
+	err := proj.ValidateLayout()
+	if err == nil || !strings.Contains(err.Error(), "paths.build must be a directory") {
+		t.Fatalf("expected paths.build-must-be-a-directory error, got %v", err)
+	}
+}
+
 func TestValidateLayoutRejectsInvalidTopDataPackageOutputNames(t *testing.T) {
 	root := t.TempDir()
 	mkdirAll(t, filepath.Join(root, "src"))