Implement the modern user-settings service and migration path

## Objective

Implement load/save/upgrade/reset and change-notification behavior with deterministic storage, serialization, versioning, concurrency, corruption handling, and privacy. Provide a practical migration path from generated settings without forcing existing applications to migrate.

## 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.

Start by locating the existing generated-settings implementation and its load/save entry points, then map the migration and notification behavior. Review the relevant tests and documentation before choosing an approach. Done means load, save, upgrade, reset, notifications, deterministic serialization, versioning, concurrency, corruption handling, privacy, and a non-forced migration path are covered and verified locally and in CI.

Written by the indexing model from the issue text.