Introduce BootPipeline and BootStage Abstractions

### Scope
- Add `BootPipeline`
- Add `BootStage` interface
- No refactor yet — infrastructure only

### Tasks
- Create `BootStage` interface
- Create `BootPipeline` executor
- Add minimal tests

### Acceptance Criteria
- BootPipeline can run ordered stages
- No existing behavior changed