feat(12.2-03): add hasMany relation contracts, relation forms and child create

- RelationContract gains Kind (empty is belongsToMany) and ForeignKey, with kind-aware boot checks
- manage.form, view.form and pivot.form compile against the related or pivot model; $/ paths resolve inside the plugin
- view toolbarButtons accept create|update|delete|link|unlink, each the capability of its routes
- POST .../relations/{name}/records creates a child through the manage form; the server sets the hasMany key
- relation schema carries kind, deferrable and the localized forms; 17 new relation message keys in en and pl

modules/cabana/relation_child_smoke_test.go

@@ -0,0 +1,80 @@
+package cabana_test
+
+import (
+	"encoding/json"
+	"fmt"
+	"net/http"
+	"slices"
+	"testing"
+)
+
+// linkedIDs lists the ids of a gadget relation's linked rows.
+func (e *conformEnv) linkedIDs(t *testing.T, gadget uint, relation string, headers map[string]string) []uint {
+	t.Helper()
+	rec := e.sendWith(t, http.MethodGet, fmt.Sprintf("/acme/conform/gadgets/%d/relations/%s", gadget, relation), nil, "", headers)
+	if rec.Code != http.StatusOK {
+		t.Fatalf("linked %s status=%d body=%s", relation, rec.Code, rec.Body.String())
+	}
+	var body struct {
+		Data []struct {
+			ID uint `json:"id"`
+		} `json:"data"`
+	}
+	if err := json.Unmarshal(rec.Body.Bytes(), &body); err != nil {
+		t.Fatal(err)
+	}
+	out := make([]uint, 0, len(body.Data))
+	for _, row := range body.Data {
+		out = append(out, row.ID)
+	}
+	return out
+}
+
+// partOwner reads a part's gadget_id straight from the table.
+func (e *conformEnv) partOwner(t *testing.T, id uint) *uint {
+	t.Helper()
+	var part conformPart
+	if err := e.db.First(&part, id).Error; err != nil {
+		t.Fatal(err)
+	}
+	return part.GadgetID
+}
+
+// TestRelationChildSmokeCreate creates a hasMany child of a saved gadget
+// through the relation manager: the server sets the foreign key from the
+// scoped parent, the child lists under that parent only, and a body naming
+// the foreign key cannot move it.
+func TestRelationChildSmokeCreate(t *testing.T) {
+	env := newConformEnv(t)
+	env.loginAs(t, env.login)
+	a := env.createGadget(t, "a-"+env.stamp, "")
+	b := env.createGadget(t, "b-"+env.stamp, "")
+
+	rec := env.send(t, http.MethodPost, fmt.Sprintf("/acme/conform/gadgets/%d/relations/parts/records", a), map[string]any{"label": "wheel", "gadget_id": b}, true)
+	if rec.Code != http.StatusCreated {
+		t.Fatalf("create part status=%d body=%s", rec.Code, rec.Body.String())
+	}
+	part := dataID(t, rec.Body.Bytes())
+	if owner := env.partOwner(t, part); owner == nil || *owner != a {
+		t.Fatalf("part gadget_id = %v, want %d", owner, a)
+	}
+	if got := env.linkedIDs(t, a, "parts", nil); !slices.Contains(got, part) {
+		t.Fatalf("gadget A parts = %v, want %d", got, part)
+	}
+	if got := env.linkedIDs(t, b, "parts", nil); slices.Contains(got, part) {
+		t.Fatalf("gadget B lists A's part: %v", got)
+	}
+
+	invalid := env.send(t, http.MethodPost, fmt.Sprintf("/acme/conform/gadgets/%d/relations/parts/records", a), map[string]any{"label": ""}, true)
+	if invalid.Code != http.StatusUnprocessableEntity {
+		t.Fatalf("empty label status=%d body=%s", invalid.Code, invalid.Body.String())
+	}
+	undeclared := env.send(t, http.MethodPost, fmt.Sprintf("/acme/conform/gadgets/%d/relations/members/records", a), map[string]any{"email": "x@example.test"}, true)
+	if undeclared.Code != http.StatusForbidden {
+		t.Fatalf("create on a relation without the create button status=%d body=%s", undeclared.Code, undeclared.Body.String())
+	}
+	missing := env.send(t, http.MethodPost, fmt.Sprintf("/acme/conform/gadgets/%d/relations/parts/records", b+1000), map[string]any{"label": "x"}, true)
+	if missing.Code != http.StatusNotFound {
+		t.Fatalf("create under a missing parent status=%d body=%s", missing.Code, missing.Body.String())
+	}
+}