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/openapi_conformance_test.go

@@ -222,6 +222,9 @@ func TestPhase10OpenAPIConformance(t *testing.T) {
 		{"POST /{vendor}/{plugin}/{controller}/{id}/relations/{name}/unlink", 200, "cabana.Envelope-cabana_RelationMutationResult", func(t *testing.T, e *conformEnv) *httptest.ResponseRecorder {
 			return e.send(t, http.MethodPost, fmt.Sprintf("/acme/conform/gadgets/%d/relations/members/unlink", e.gadgetID), map[string]any{"ids": []uint{e.memberID}}, true)
 		}, into[cabana.Envelope[cabana.RelationMutationResult]](), nil},
+		{"POST /{vendor}/{plugin}/{controller}/{id}/relations/{name}/records", 201, "cabana.RecordEnvelope", func(t *testing.T, e *conformEnv) *httptest.ResponseRecorder {
+			return e.send(t, http.MethodPost, fmt.Sprintf("/acme/conform/gadgets/%d/relations/parts/records", e.gadgetID), map[string]any{"label": "part-" + e.stamp}, true)
+		}, into[cabana.RecordEnvelope](), nil},
 		{"POST /{vendor}/{plugin}/{controller}/toolbar/{action}", 200, "cabana.Envelope-cabana_AdminActionResult", func(t *testing.T, e *conformEnv) *httptest.ResponseRecorder {
 			return e.send(t, http.MethodPost, "/acme/conform/gadgets/toolbar/recount", map[string]any{}, true)
 		}, into[cabana.Envelope[cabana.AdminActionResult]](), nil},
@@ -474,7 +477,7 @@ func dataID(t *testing.T, raw []byte) uint {
 func newConformEnv(t *testing.T) *conformEnv {
 	t.Helper()
 	gdb := adminGorm(t)
-	models := []any{&conformGadget{}, &conformGroup{}, &conformMember{}, &conformGadgetMember{}, &conformSettings{}}
+	models := []any{&conformGadget{}, &conformGroup{}, &conformMember{}, &conformGadgetMember{}, &conformPart{}, &conformSettings{}}
 	if err := gdb.Migrator().DropTable(models...); err != nil {
 		t.Fatal(err)
 	}
@@ -543,6 +546,7 @@ type conformGadget struct {
 	ReleasedOn lagoon.Date     `gorm:"column:released_on;type:date"`
 	StartsAt   *time.Time      `gorm:"column:starts_at"`
 	Members    []conformMember `gorm:"-"`
+	Parts      []conformPart   `gorm:"-"`
 	CreatedAt  time.Time       `gorm:"column:created_at"`
 	UpdatedAt  time.Time       `gorm:"column:updated_at"`
 }
@@ -586,6 +590,20 @@ type conformGadgetMember struct {
 
 func (conformGadgetMember) TableName() string { return "cabana_conform_gadget_members" }
 
+// conformPart is the hasMany child of a gadget; its nullable gadget_id makes
+// the parts relation deferrable.
+type conformPart struct {
+	ID        uint      `gorm:"column:id;primaryKey"`
+	GadgetID  *uint     `gorm:"column:gadget_id"`
+	Label     string    `gorm:"column:label"`
+	CreatedAt time.Time `gorm:"column:created_at"`
+	UpdatedAt time.Time `gorm:"column:updated_at"`
+}
+
+func (conformPart) TableName() string        { return "cabana_conform_parts" }
+func (conformPart) Fillable() []string       { return []string{"label"} }
+func (conformPart) Rules() map[string]string { return map[string]string{"label": "required"} }
+
 type conformSettings struct {
 	ID      uint `gorm:"column:id;primaryKey"`
 	Enabled bool `gorm:"column:enabled"`
@@ -630,6 +648,9 @@ func (conformController) AdminRelationContracts() []cabana.RelationContract {
 	return []cabana.RelationContract{{
 		Name: "members", NewRelated: func() any { return &conformMember{} }, NewPivot: func() any { return &conformGadgetMember{} },
 		ParentForeignKey: "gadget_id", RelatedForeignKey: "member_id", Columns: map[string]string{"email": "email"},
+	}, {
+		Name: "parts", Kind: cabana.RelationHasMany, NewRelated: func() any { return &conformPart{} },
+		ForeignKey: "gadget_id", Columns: map[string]string{"label": "label"},
 	}}
 }
 func (conformController) AdminFieldRelations() []cabana.FieldRelationContract {
@@ -748,6 +769,20 @@ update:
                 email:
                     label: Email
         showSearch: true
+parts:
+    label: Parts
+    view:
+        list:
+            columns:
+                label:
+                    label: Label
+        toolbarButtons: create
+    manage:
+        form: $/acme/conform/models/part/fields.yaml
+        list:
+            columns:
+                label:
+                    label: Label
 `),
 		"models/gadget/columns.yaml": file(`columns:
     name:
@@ -779,6 +814,9 @@ update:
         type: relation-manager
         relation: members
         context: [update]
+    parts:
+        type: relation-manager
+        relation: parts
     lookup:
         label: Lookup
         type: widget
@@ -814,6 +852,12 @@ update:
         fileTypes: [pdf, png, svg, txt]
         useCaption: true
         context: update
+`),
+		"models/part/fields.yaml": file(`fields:
+    label:
+        label: Label
+        type: text
+        required: true
 `),
 		"models/settings/fields.yaml": file(`fields:
     enabled: