[Application Builder] Establish the WinForms host and application lifetime

## User story

As a WinForms developer, I want a first-class application builder and host lifetime so that startup, the UI thread, application context, and graceful shutdown follow modern .NET conventions without hiding WinForms' desktop semantics.

## Rationale and user benefit

This is the foundation for the remaining Application Builder work. It should align with the Generic Host and existing proposal #11415 while retaining predictable `Application.Run`, `ApplicationContext`, synchronization-context, exception, and shutdown behavior.

## Scope boundary

Own the core builder, built application, options, host/lifetime integration, UI-thread startup, application context, startup-form activation, and graceful shutdown contracts. Coordinate public contracts with the DI/scopes and services/configuration stories, but do not implement their internal features here.

## Acceptance criteria

- [ ] C# and VB can configure, build, run, and stop the same application model.
- [ ] Startup form and custom `ApplicationContext` scenarios are supported.
- [ ] UI-thread and synchronization-context behavior is explicit and deterministic.
- [ ] Host and WinForms shutdown signals propagate correctly in both directions.
- [ ] Existing `Application.Run` applications remain supported and source compatible.
- [ ] The relationship with #11415 is resolved without competing lifetime models.

## Definition of Done

- [ ] API shape and ownership boundaries are reviewed.
- [ ] Runtime implementation, automated tests, samples, diagnostics, and compatibility documentation are complete.
- [ ] Startup/shutdown failure paths, resource cleanup, performance, and accessibility implications are validated.
- [ ] CI passes on supported architectures.

## Parallelization

This is foundation stream A. Stabilize the minimum shared contracts early so streams B and C can work against them without editing the same implementation areas.

Start by reading proposal #11415 and the existing Application.Run and ApplicationContext behavior referenced in the issue. Define the shared builder, host, lifetime, startup-form, and shutdown contracts before coordinating with the DI/scopes and services/configuration work. Done means reviewed API boundaries, implementation, tests, samples, diagnostics, compatibility documentation, and passing CI.

Written by the indexing model from the issue text.