Implement shared startup, shutdown, exception, and command-line features

## Objective

Implement approved shared lifecycle features using the common host rather than a parallel message-loop model. Preserve deterministic event ordering, UI-thread behavior, cancellation, exception routing, command-line parsing, settings-on-exit policy, and graceful shutdown.

## Completion guidance

- Keep the implementation focused on the objective while allowing the assignee to improve the approach based on research.
- Document important decisions, rejected alternatives, dependencies, and follow-up work.
- Add or update tests and documentation appropriate to the work.
- Verify locally and in CI where the task changes code or generated artifacts.

No files, tests, or concrete entry points are named. Start by locating the common host and the existing message-loop and lifecycle implementations, then trace startup, shutdown, exception, cancellation, command-line, and settings-on-exit behavior; done means the shared implementation preserves deterministic ordering and UI-thread behavior with appropriate tests and documentation.

Written by the indexing model from the issue text.