Define Application Execution Boundary in DI

[armanist]

Scope

• Make Di::reset() the official application execution boundary
• Ensure it is called exactly once per App execution
• No behavioral changes yet

Tasks

• Document Di::reset() as the execution boundary
• Call Di::reset() in AppFactory::create()
• Ensure no mid-execution resets exist

Acceptance Criteria

• One DI container per App execution
• No static instances survive between runs