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>

examples/hello/hello_test.go

@@ -2,6 +2,7 @@ package main
 
 import (
 	"bytes"
+	"context"
 	"crypto/sha256"
 	"net/http"
 	"net/http/httptest"
@@ -18,6 +19,7 @@ import (
 	"git.golem15.com/golem15/summercms/phrasebook"
 	"git.golem15.com/golem15/summercms/postcard"
 	"git.golem15.com/golem15/summercms/surf"
+	"git.golem15.com/golem15/summercms/towel"
 )
 
 func TestGreeterHelloPrintsLayeredConfig(t *testing.T) {
@@ -122,6 +124,13 @@ func TestHelloPluginResolvesLang(t *testing.T) {
 	if !ok {
 		t.Fatal("translator not published")
 	}
+	ctx := towel.WithLocale(context.Background(), "pl")
+	if got := tr.Get(ctx, "golem15.hello::lang.greeting", nil); got != "Cześć" {
+		t.Fatalf("towel locale = %q", got)
+	}
+	if got := tr.Get(context.Background(), "golem15.hello::lang.greeting", nil); got != "Hello" {
+		t.Fatalf("app locale default = %q", got)
+	}
 	if got := tr.GetIn("en", "golem15.hello::lang.labels.pluginName", nil); got != "Hello plugin" {
 		t.Fatalf("en = %q", got)
 	}