refactor: decouple executor from storage layer, extract remaining BSON

Replace *mpr.Reader/*mpr.Writer with backend.FullBackend throughout
executor. Inject BackendFactory to remove mprbackend import from
executor_connect.go. Move all remaining write-path BSON construction
(DataGrid2, filters, cloning, widget property updates) behind backend
interface.

- Remove writer/reader fields from Executor struct
- Add BackendFactory injection pattern for connect/disconnect
- Create mdl/backend/mpr/datagrid_builder.go (1260 lines)
- Add BuildDataGrid2Widget, BuildFilterWidget to WidgetBuilderBackend
- Delete bson_helpers.go, cmd_pages_builder_input_cloning.go,
  cmd_pages_builder_input_datagrid.go, cmd_pages_builder_v3_pluggable.go
- Remaining BSON: 3 read-only files (describe, diff) — WASM-safe

Author: retran

cmd/mxcli/main.go

@@ -9,6 +9,8 @@ import (
 	"os"
 	"strings"
 
+	"github.com/mendixlabs/mxcli/mdl/backend"
+	mprbackend "github.com/mendixlabs/mxcli/mdl/backend/mpr"
 	"github.com/mendixlabs/mxcli/mdl/diaglog"
 	"github.com/mendixlabs/mxcli/mdl/executor"
 	"github.com/mendixlabs/mxcli/mdl/repl"
@@ -194,6 +196,7 @@ func resolveFormat(cmd *cobra.Command, defaultFormat string) string {
 func newLoggedExecutor(mode string) (*executor.Executor, *diaglog.Logger) {
 	logger := diaglog.Init(version, mode)
 	exec := executor.New(os.Stdout)
+	exec.SetBackendFactory(func() backend.FullBackend { return mprbackend.New() })
 	exec.SetLogger(logger)
 	if globalJSONFlag {
 		exec.SetFormat(executor.FormatJSON)

examples/create_datagrid2_page/main.go

@@ -22,6 +22,8 @@ import (
 	"os"
 	"strings"
 
+	"github.com/mendixlabs/mxcli/mdl/backend"
+	mprbackend "github.com/mendixlabs/mxcli/mdl/backend/mpr"
 	"github.com/mendixlabs/mxcli/mdl/executor"
 	"github.com/mendixlabs/mxcli/mdl/visitor"
 )
@@ -51,6 +53,7 @@ func main() {
 
 	// Create the MDL executor with stdout for output
 	exec := executor.New(os.Stdout)
+	exec.SetBackendFactory(func() backend.FullBackend { return mprbackend.New() })
 
 	// Define the MDL script to create a page with DataGrid2
 	// Note: Adjust module name, entity name, and attributes to match your project

mdl/backend/mock/backend.go

@@ -276,6 +276,8 @@ type MockBackend struct {
 	SerializeWidgetToOpaqueFunc     func(w pages.Widget) any
 	SerializeDataSourceToOpaqueFunc func(ds pages.DataSource) any
 	BuildCreateAttributeObjectFunc  func(attributePath string, objectTypeID, propertyTypeID, valueTypeID string) (any, error)
+	BuildDataGrid2WidgetFunc        func(id model.ID, name string, spec backend.DataGridSpec, projectPath string) (*pages.CustomWidget, error)
+	BuildFilterWidgetFunc           func(spec backend.FilterWidgetSpec, projectPath string) (pages.Widget, error)
 
 	// AgentEditorBackend
 	ListAgentEditorModelsFunc               func() ([]*agenteditor.Model, error)

mdl/backend/mock/mock_mutation.go

@@ -94,3 +94,17 @@ func (m *MockBackend) BuildCreateAttributeObject(attributePath string, objectTyp
 	}
 	return nil, nil
 }
+
+func (m *MockBackend) BuildDataGrid2Widget(id model.ID, name string, spec backend.DataGridSpec, projectPath string) (*pages.CustomWidget, error) {
+	if m.BuildDataGrid2WidgetFunc != nil {
+		return m.BuildDataGrid2WidgetFunc(id, name, spec, projectPath)
+	}
+	return nil, nil
+}
+
+func (m *MockBackend) BuildFilterWidget(spec backend.FilterWidgetSpec, projectPath string) (pages.Widget, error) {
+	if m.BuildFilterWidgetFunc != nil {
+		return m.BuildFilterWidgetFunc(spec, projectPath)
+	}
+	return nil, nil
+}

mdl/backend/mpr/backend.go

@@ -34,6 +34,11 @@ type MprBackend struct {
 	path   string
 }
 
+// New creates a new unconnected MprBackend. Call Connect(path) to open a project.
+func New() *MprBackend {
+	return &MprBackend{}
+}
+
 // Wrap creates an MprBackend that wraps an existing Writer (and its Reader).
 // This is used during migration when the Executor already owns the Writer
 // and we want to expose it through the Backend interface without opening
@@ -75,6 +80,11 @@ func (b *MprBackend) Disconnect() error {
 func (b *MprBackend) IsConnected() bool { return b.writer != nil }
 func (b *MprBackend) Path() string      { return b.path }
 
+// MprReader returns the underlying *mpr.Reader for callers that still
+// require direct SDK access (e.g. linter rules). Prefer Backend methods
+// for new code.
+func (b *MprBackend) MprReader() *mpr.Reader { return b.reader }
+
 func (b *MprBackend) Version() types.MPRVersion                 { return convertMPRVersion(b.reader.Version()) }
 func (b *MprBackend) ProjectVersion() *types.ProjectVersion     { return convertProjectVersion(b.reader.ProjectVersion()) }
 func (b *MprBackend) GetMendixVersion() (string, error)         { return b.reader.GetMendixVersion() }