Build host-lifetime samples and an exploratory validation matrix

## Objective

Create small C# and VB samples for the default startup form, custom `ApplicationContext`, background service, cancellation, and graceful shutdown. Validate local and Terminal Services sessions, x64 and Windows on ARM, light and Dark Mode, and slow hardware where lifecycle responsiveness may differ.

## 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 locating the existing WinForms sample and test areas, then scope samples for the default startup form, custom ApplicationContext, background service, cancellation, and graceful shutdown. Done means the C# and VB samples and exploratory matrix cover the listed session, architecture, theme, and hardware conditions, with decisions and follow-up work documented.

Written by the indexing model from the issue text.