Define and prototype the Designer/runtime DI contract

[KlausLoeffelmann]

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.