feat(04-02): load namespaced translations from embedded plugins

- Walk HasLang FS for lang/<locale>/<group>.yaml and flatten nested keys
- Publish one phrasebook.Translator before Boot with en/en defaults
- Hello plugin resolves en/pl vendor.plugin::group.key after Activate

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

party/registry_test.go

@@ -1,6 +1,7 @@
 package party
 
 import (
+	"fmt"
 	"io/fs"
 	"os"
 	"path/filepath"
@@ -12,6 +13,7 @@ import (
 	"git.golem15.com/golem15/summercms/bonfire"
 	"git.golem15.com/golem15/summercms/compass"
 	"git.golem15.com/golem15/summercms/pact"
+	"git.golem15.com/golem15/summercms/phrasebook"
 )
 
 type tracePlugin struct {
@@ -394,3 +396,62 @@ func TestActivateDiscoversHasConfigAndHasCommands(t *testing.T) {
 		t.Fatal("plain plugin must not satisfy HasConfig")
 	}
 }
+
+type langPlugin struct {
+	id   string
+	fsys fs.FS
+	got  *string
+}
+
+func (p *langPlugin) ID() string                   { return p.id }
+func (p *langPlugin) Requires() []string           { return nil }
+func (p *langPlugin) Register(*backpack.App) error { return nil }
+func (p *langPlugin) LangFS() fs.FS                { return p.fsys }
+
+func (p *langPlugin) Boot(app *backpack.App) error {
+	if p.got == nil {
+		return nil
+	}
+	tr, ok := app.Lookup[*phrasebook.Translator]()
+	if !ok {
+		return fmt.Errorf("translator missing at boot")
+	}
+	*p.got = tr.GetIn("en", p.id+"::lang.greeting", nil)
+	return nil
+}
+
+func TestActivatePublishesTranslatorBeforeBoot(t *testing.T) {
+	var got string
+	p := &langPlugin{
+		id:  "golem15.hello",
+		got: &got,
+		fsys: fstest.MapFS{
+			"lang/en/lang.yaml": {Data: []byte("greeting: Hello\n")},
+		},
+	}
+	app := backpack.New(&compass.Config{})
+	if _, err := activate([]Plugin{p}, app, []string{"golem15.hello"}); err != nil {
+		t.Fatalf("activate: %v", err)
+	}
+	if got != "Hello" {
+		t.Fatalf("boot lookup = %q, want Hello (translator must be published before Boot)", got)
+	}
+}
+
+func TestActivateMalformedLangFailsWithPluginContext(t *testing.T) {
+	p := &langPlugin{
+		id: "golem15.hello",
+		fsys: fstest.MapFS{
+			"lang/en.yaml": {Data: []byte("greeting: Hello\n")},
+		},
+	}
+	app := backpack.New(&compass.Config{})
+	_, err := activate([]Plugin{p}, app, []string{"golem15.hello"})
+	if err == nil {
+		t.Fatal("expected malformed catalog error")
+	}
+	msg := err.Error()
+	if !strings.Contains(msg, "golem15.hello") || !strings.Contains(msg, "malformed path") {
+		t.Fatalf("error %q should name the plugin and path problem", msg)
+	}
+}