refactor: implement mutation backends and migrate handlers off sdk/mpr

Implement PageMutator, WorkflowMutator, and WidgetBuilderBackend in
mdl/backend/mpr/. Rewrite ALTER PAGE (1721→256 lines) and ALTER
WORKFLOW (887→178 lines) as thin orchestrators using mutator sessions.
Implement PluggableWidgetEngine with WidgetObjectBuilder interface,
eliminating all BSON from widget_engine.go.

- Create mdl/backend/mpr/page_mutator.go (1554 lines)
- Create mdl/backend/mpr/workflow_mutator.go (771 lines)
- Create mdl/backend/mpr/widget_builder.go (1007 lines)
- Migrate SerializeWidget/ClientAction/DataSource to backend interface
- Add ParseMicroflowFromRaw to MicroflowBackend interface
- Delete widget_operations.go, widget_templates.go, widget_defaults.go
- Move ALTER PAGE/WORKFLOW tests to backend/mpr/ package

Author: retran

mdl/backend/backend.go

@@ -34,4 +34,5 @@ type FullBackend interface {
 	PageMutationBackend
 	WorkflowMutationBackend
 	WidgetSerializationBackend
+	WidgetBuilderBackend
 }

mdl/backend/microflow.go

@@ -16,6 +16,10 @@ type MicroflowBackend interface {
 	DeleteMicroflow(id model.ID) error
 	MoveMicroflow(mf *microflows.Microflow) error
 
+	// ParseMicroflowFromRaw builds a Microflow from an already-unmarshalled
+	// BSON map. Used by diff-local and other callers that have raw map data.
+	ParseMicroflowFromRaw(raw map[string]any, unitID, containerID model.ID) *microflows.Microflow
+
 	ListNanoflows() ([]*microflows.Nanoflow, error)
 	GetNanoflow(id model.ID) (*microflows.Nanoflow, error)
 	CreateNanoflow(nf *microflows.Nanoflow) error

mdl/backend/mock/backend.go

@@ -80,7 +80,8 @@ type MockBackend struct {
 	CreateMicroflowFunc func(mf *microflows.Microflow) error
 	UpdateMicroflowFunc func(mf *microflows.Microflow) error
 	DeleteMicroflowFunc func(id model.ID) error
-	MoveMicroflowFunc   func(mf *microflows.Microflow) error
+	MoveMicroflowFunc          func(mf *microflows.Microflow) error
+	ParseMicroflowFromRawFunc  func(raw map[string]any, unitID, containerID model.ID) *microflows.Microflow
 	ListNanoflowsFunc   func() ([]*microflows.Nanoflow, error)
 	GetNanoflowFunc     func(id model.ID) (*microflows.Nanoflow, error)
 	CreateNanoflowFunc  func(nf *microflows.Nanoflow) error
@@ -270,6 +271,12 @@ type MockBackend struct {
 	SerializeDataSourceFunc       func(ds pages.DataSource) (any, error)
 	SerializeWorkflowActivityFunc func(a workflows.WorkflowActivity) (any, error)
 
+	// WidgetBuilderBackend
+	LoadWidgetTemplateFunc          func(widgetID string, projectPath string) (backend.WidgetObjectBuilder, error)
+	SerializeWidgetToOpaqueFunc     func(w pages.Widget) any
+	SerializeDataSourceToOpaqueFunc func(ds pages.DataSource) any
+	BuildCreateAttributeObjectFunc  func(attributePath string, objectTypeID, propertyTypeID, valueTypeID string) (any, error)
+
 	// AgentEditorBackend
 	ListAgentEditorModelsFunc               func() ([]*agenteditor.Model, error)
 	ListAgentEditorKnowledgeBasesFunc       func() ([]*agenteditor.KnowledgeBase, error)

mdl/backend/mock/mock_microflow.go

@@ -49,6 +49,13 @@ func (m *MockBackend) MoveMicroflow(mf *microflows.Microflow) error {
 	return nil
 }
 
+func (m *MockBackend) ParseMicroflowFromRaw(raw map[string]any, unitID, containerID model.ID) *microflows.Microflow {
+	if m.ParseMicroflowFromRawFunc != nil {
+		return m.ParseMicroflowFromRawFunc(raw, unitID, containerID)
+	}
+	return nil
+}
+
 func (m *MockBackend) ListNanoflows() ([]*microflows.Nanoflow, error) {
 	if m.ListNanoflowsFunc != nil {
 		return m.ListNanoflowsFunc()

mdl/backend/mock/mock_mutation.go

@@ -62,3 +62,35 @@ func (m *MockBackend) SerializeWorkflowActivity(a workflows.WorkflowActivity) (a
 	}
 	return nil, nil
 }
+
+// ---------------------------------------------------------------------------
+// WidgetBuilderBackend
+// ---------------------------------------------------------------------------
+
+func (m *MockBackend) LoadWidgetTemplate(widgetID string, projectPath string) (backend.WidgetObjectBuilder, error) {
+	if m.LoadWidgetTemplateFunc != nil {
+		return m.LoadWidgetTemplateFunc(widgetID, projectPath)
+	}
+	return nil, nil
+}
+
+func (m *MockBackend) SerializeWidgetToOpaque(w pages.Widget) any {
+	if m.SerializeWidgetToOpaqueFunc != nil {
+		return m.SerializeWidgetToOpaqueFunc(w)
+	}
+	return nil
+}
+
+func (m *MockBackend) SerializeDataSourceToOpaque(ds pages.DataSource) any {
+	if m.SerializeDataSourceToOpaqueFunc != nil {
+		return m.SerializeDataSourceToOpaqueFunc(ds)
+	}
+	return nil
+}
+
+func (m *MockBackend) BuildCreateAttributeObject(attributePath string, objectTypeID, propertyTypeID, valueTypeID string) (any, error) {
+	if m.BuildCreateAttributeObjectFunc != nil {
+		return m.BuildCreateAttributeObjectFunc(attributePath, objectTypeID, propertyTypeID, valueTypeID)
+	}
+	return nil, nil
+}

mdl/backend/mpr/backend.go

@@ -211,6 +211,9 @@ func (b *MprBackend) MoveMicroflow(mf *microflows.Microflow) error {
 func (b *MprBackend) ListNanoflows() ([]*microflows.Nanoflow, error) {
 	return b.reader.ListNanoflows()
 }
+func (b *MprBackend) ParseMicroflowFromRaw(raw map[string]any, unitID, containerID model.ID) *microflows.Microflow {
+	return mpr.ParseMicroflowFromRaw(raw, unitID, containerID)
+}
 func (b *MprBackend) GetNanoflow(id model.ID) (*microflows.Nanoflow, error) {
 	return b.reader.GetNanoflow(id)
 }
@@ -726,17 +729,17 @@ func (b *MprBackend) DeleteAgentEditorAgent(id string) error {
 }
 
 // ---------------------------------------------------------------------------
-// PageMutationBackend
+// PageMutationBackend — implemented in page_mutator.go
+// ---------------------------------------------------------------------------
 
-func (b *MprBackend) OpenPageForMutation(unitID model.ID) (backend.PageMutator, error) {
-	panic("MprBackend.OpenPageForMutation not yet implemented") // TODO: implement in PR #237
-}
+// OpenPageForMutation is implemented in page_mutator.go.
 
 // ---------------------------------------------------------------------------
 // WorkflowMutationBackend
 
+// OpenWorkflowForMutation is implemented in workflow_mutator.go.
 func (b *MprBackend) OpenWorkflowForMutation(unitID model.ID) (backend.WorkflowMutator, error) {
-	panic("MprBackend.OpenWorkflowForMutation not yet implemented") // TODO: implement in PR #237
+	return b.openWorkflowForMutation(unitID)
 }
 
 // ---------------------------------------------------------------------------
@@ -751,7 +754,7 @@ func (b *MprBackend) SerializeClientAction(a pages.ClientAction) (any, error) {
 }
 
 func (b *MprBackend) SerializeDataSource(ds pages.DataSource) (any, error) {
-	panic("MprBackend.SerializeDataSource not yet implemented") // TODO: implement in PR #237
+	return mpr.SerializeCustomWidgetDataSource(ds), nil
 }
 
 func (b *MprBackend) SerializeWorkflowActivity(a workflows.WorkflowActivity) (any, error) {