Prototype the core WinForms application builder and lifetime contracts

## 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.

Start by mapping the existing WinForms startup-form/application-context activation and lifetime-service paths, then review how a narrow builder, built application, options, and lifetime contracts could fit without pre-empting DI, scopes, or configuration. Document decisions, rejected alternatives, dependencies, and follow-up work; add appropriate tests and documentation, then verify locally and in CI.

Written by the indexing model from the issue text.