Research Designer-safe service assignment and UI scope ownership

## Objective

Validate whether a marker/assignment contract, generated hookup code, activation factory, or another pattern best bridges parameterless Designer creation and runtime DI. Include Forms, UserControls, components, inherited forms, nested controls, modal dialogs, and out-of-process Designer constraints. Define scope ownership and disposal rules.

## 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.

No files, tests, or entry points are named. Start by researching the Designer/runtime DI boundary across Forms, UserControls, components, inherited forms, nested controls, modal dialogs, and out-of-process Designer constraints; done means documented decisions, rejected alternatives, scope and disposal rules, appropriate tests and documentation, and local/CI verification.

Written by the indexing model from the issue text.