Define and prototype the Designer/runtime DI contract

## Objective

Prototype the smallest contract for assigning or obtaining services at runtime while keeping design-time creation valid. Establish the boundary between runtime, Designer code generation, component model, and application host. Avoid committing to the provisional `IServiceProviderAssignable` shape without review.

## 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.

Start by mapping the boundary between runtime, Designer code generation, the component model, and the application host. Review the provisional IServiceProviderAssignable shape before defining the smallest contract. Done means a reviewed prototype, documented decisions and rejected alternatives, appropriate tests and documentation, and local and CI verification.

Written by the indexing model from the issue text.