[Application Builder] Enable Designer-safe dependency injection and UI scopes

## User story

As a WinForms developer, I want Forms, UserControls, and components to participate in dependency injection and well-defined UI scopes without breaking drag-and-drop Designer workflows.

## Rationale and user benefit

Constructor injection conflicts with parameterless design-time creation. Static service locators hide lifetime errors and make testing harder. A supported bridge must keep the out-of-process Designer, generated code, runtime activation, disposal, and scope ownership coherent.

## Scope boundary

Own Designer/runtime service assignment, activation and factory contracts, Form/UserControl/component scopes, disposal, and scope-aware navigation. Depend only on the minimum host contracts from the lifetime story. Do not own configuration providers, logging providers, or project templates.

## Acceptance criteria

- [ ] Drag-and-drop and generated Designer code continue to work.
- [ ] Runtime-created Forms and controls can resolve required services without a global static provider.
- [ ] Scope ownership and disposal are deterministic for Forms, modal dialogs, UserControls, and application contexts.
- [ ] Design-time service absence is explicit and does not leak into runtime behavior.
- [ ] Existing controls and components require no migration.
- [ ] Out-of-process Designer requirements are agreed with the Designer team.

## Definition of Done

- [ ] Public and Designer contracts are reviewed together.
- [ ] Runtime and Designer implementations, tests, samples, and migration guidance are complete.
- [ ] Disposal, nested scopes, exception paths, design-time detection, memory, and handle stability are validated.

## Parallelization

This is foundation stream B. Keep Designer/code-generation changes isolated from stream A's lifetime implementation and stream C's service integrations.

No files, tests, or entry points are named. Start by reviewing the Designer/runtime service-assignment, activation, factory, and scope requirements, then coordinate the out-of-process Designer contract with the Designer team; done means reviewed public contracts, implementations, tests, samples, migration guidance, and validated disposal, nested scopes, design-time detection, memory, and handle stability.

Written by the indexing model from the issue text.