Prototype the core WinForms application builder and lifetime contracts

[KlausLoeffelmann]

Objective

Create a reviewable prototype for the builder, built application, options, startup-form/application-context activation, and lifetime service. Keep the implementation narrow and leave extension points for DI/scopes and configuration/services rather than pre-empting those stories.

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.