Implement and validate out-of-process Designer integration

## Objective

Coordinate the Designer-side recognition and generated-code behavior for the approved contract. Cover drag-and-drop, serialization, inherited designs, rename/remove operations, undo/redo, reload, and applications that do not use the builder.

## Completion guidance

- Keep the implementation focused on the objective while allowing the assignee to improve the approach based on research.
- Document important decisions, rejected alternatives, dependencies, and follow-up work.
- Add or update tests and documentation appropriate to the work.
- Verify locally and in CI where the task changes code or generated artifacts.

The issue names no files, tests, or entry points. Start by locating the approved contract and the Designer-side recognition and generated-code integration points, then map the required scenarios: drag-and-drop, serialization, inherited designs, rename/remove, undo/redo, reload, and applications without the builder. Done means the behavior is implemented and validated, with appropriate tests, documentation, decision records, and local and CI verification.

Written by the indexing model from the issue text.