Add runtime object editor example with screen/object DnD modeling and metadata mutation

[Copilot]

This PR introduces a new x0 example app that acts as an in-browser editor for building and restructuring screens at runtime. It adds a custom editor object that can create/edit/reparent UI objects while keeping object.json and skeleton.json runtime structures synchronized.

• New example application (example16)

  • Added /example/object_editor with sql/, static/, and README.md
  • Registered app config with template_file and setup_class so the custom editor object is loadable from metadata
  • Updated top-level example documentation to include the new example

• Custom editor object type (ObjectEditor)

  • Added sysObjObjectEditor.js as a template-loaded custom x0 object
  • Implements:
    • main object bar:
      • screen section (add/remove/switch)
      • system-object section (all registered setup classes)
    • properties form for selected object
    • workspace canvas for hierarchy visualization and drop targets

• Runtime model mutation behavior

  • New-object creation updates:
    • sysFactory.DataObject.XMLRPCResultData (object definitions)
    • sysFactory.DataSkeleton.XMLRPCResultData (screen hierarchy refs)
  • Existing-object reparenting updates skeleton refs in place and rebuilds affected screen without full page reload
  • Screen operations (add/remove/switch) integrate directly with sysFactory.switchScreen()

• Guardrails and consistency controls

  • Unique object ID generation
  • Parent-cycle prevention during reparenting
  • Last-screen deletion protection
  • Safe status rendering (text sink, not HTML sink)

• Integration test coverage scaffold

  • Added dedicated test config under /test/integration/config/editor_builder
  • Added test/integration/test_editor_builder.py to exercise editor flows (screen add/switch + runtime add/reparent assertions)

// runtime object + skeleton mutation during create
this.getObjectStore()[ObjectID] = {
  'Type': TypeID,
  'Attributes': Attributes
};

const NewEntry = {};
NewEntry[ObjectID] = { 'RefID': ParentID };
this.getSkeletonStore()[ScreenID].push(NewEntry);