test(05-06): assert HasHidden fields never marshal

- Fixture model with json:\"-\" keeps the sentinel out of JSON
- Registered-model walk stays in fonoteka.go so the framework stays app-agnostic

lagoon/hidden_marshal_test.go

@@ -0,0 +1,42 @@
+package lagoon
+
+import (
+	"encoding/json"
+	"strings"
+	"testing"
+)
+
+// hiddenMarshalFixture covers the HasHidden contract inside the framework
+// module. Enumerating fonoteka.go plugin models.All() from this package
+// would import the application into summercms.go, which CLAUDE.md forbids;
+// that registry walk lives in fonoteka.go (classes.TestHiddenNeverMarshals).
+type hiddenMarshalFixture struct {
+	Name         string `gorm:"column:name" json:"name"`
+	CollectionID uint   `gorm:"column:collection_id" json:"collection_id"`
+	Secret       string `gorm:"column:secret" json:"-"`
+}
+
+func (hiddenMarshalFixture) Hidden() []string { return []string{"secret"} }
+
+func TestHiddenNeverMarshals(t *testing.T) {
+	row := hiddenMarshalFixture{Name: "ok", CollectionID: 3, Secret: hiddenSentinel}
+	raw, err := json.Marshal(row)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if strings.Contains(string(raw), hiddenSentinel) {
+		t.Fatalf("hidden sentinel leaked: %s", raw)
+	}
+	var payload map[string]any
+	if err := json.Unmarshal(raw, &payload); err != nil {
+		t.Fatal(err)
+	}
+	if _, ok := payload["secret"]; ok {
+		t.Fatalf("json:\"-\" field present: %s", raw)
+	}
+	if payload["name"] != "ok" {
+		t.Fatalf("visible field missing: %s", raw)
+	}
+}
+
+const hiddenSentinel = "HIDDEN-SENTINEL-05-06"