test(04-04): close translation lookup and catalog boundary coverage

- Cover nested groups, malformed paths, invalid pipes, and compass locale
- Name plugin, file, and key on activation catalog failures
- Prove hello activation reads the translator through towel.WithLocale

Co-authored-by: Cursor <cursoragent@cursor.com>

party/registry_test.go

@@ -457,6 +457,26 @@ func TestActivateMalformedLangFailsWithPluginContext(t *testing.T) {
 	}
 }
 
+func TestActivateMalformedLangLeafNamesFileAndKey(t *testing.T) {
+	p := &langPlugin{
+		id: "golem15.hello",
+		fsys: fstest.MapFS{
+			"lang/en/lang.yaml": {Data: []byte("labels:\n  count: 1\n")},
+		},
+	}
+	app := backpack.New(&compass.Config{})
+	_, err := activate([]Plugin{p}, app, []string{"golem15.hello"})
+	if err == nil {
+		t.Fatal("expected non-string leaf error")
+	}
+	msg := err.Error()
+	for _, want := range []string{"golem15.hello", "lang/en/lang.yaml", "labels.count", "non-string leaf"} {
+		if !strings.Contains(msg, want) {
+			t.Fatalf("error %q missing %s", msg, want)
+		}
+	}
+}
+
 type mailPlugin struct {
 	id        string
 	fsys      fs.FS