fix: address PR mendixlabs#68 review + migrate filter widgets to engine

Review fixes:
- Remove 7 debug log.Printf calls + timelineCustom debug block
- deepCloneMap returns error instead of silent nil
- Mode fallback returns error for ambiguous multiple defaults
- createAttributeObject validates path format upfront
- Association mapping validates EntityName is set

Engine migration:
- Add attributeObjects operation for Attributes: [...] → BSON Objects
- Create .def.json for TEXTFILTER/NUMBERFILTER/DROPDOWNFILTER/DATEFILTER
- Delete ~200 lines of hardcoded filter builders
- Add DROPDOWNSORT full-stack (lexer/parser/describe/def.json)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: engalar

mdl/executor/cmd_pages_builder_v3.go

@@ -311,14 +311,6 @@ func (pb *pageBuilder) buildWidgetV3(w *ast.WidgetV3) (pages.Widget, error) {
 		widget, err = pb.buildTemplateV3(w)
 	case "FILTER":
 		widget, err = pb.buildFilterV3(w)
-	case "TEXTFILTER":
-		widget, err = pb.buildTextFilterV3(w)
-	case "NUMBERFILTER":
-		widget, err = pb.buildNumberFilterV3(w)
-	case "DROPDOWNFILTER":
-		widget, err = pb.buildDropdownFilterV3(w)
-	case "DATEFILTER":
-		widget, err = pb.buildDateFilterV3(w)
 	case "STATICIMAGE":
 		widget, err = pb.buildStaticImageV3(w)
 	case "DYNAMICIMAGE":