Extract Core Boot Logic from AppTrait into Boot Stages

### Scope
- Replace hidden constructor logic
- Preserve exact boot order

### Stages to Extract
- RegisterCoreDependenciesStage
- LoadHelpersStage
- LoadEnvironmentStage
- LoadConfigStage
- LoadAppConfigStage
- SetupErrorHandlerStage
- LoadLanguageStage

### Acceptance Criteria
- AppTrait no longer performs lifecycle orchestration
- Boot order matches current behavior exactly
