[design] Clarify persistence manager import and registration boundary

[huanghuoguoguo]

Background

Follow-up from #2176.

After rebasing onto current master, the original isolated import failure was not reproducible in the same form. Still, the persistence layer relies on import-time registration and application-level imports, so the intended testability/runtime boundary should be made explicit.

Questions to decide

• Should persistence database implementations be importable and testable without constructing the application graph?
• Should manager/database registration live in import-time decorators, explicit registry setup, or dependency injection?
• Is repeated registry initialization in a single Python process supported?
• Should tests assert circular-import freedom at module-import level, or only through application startup flows?

Suggested next step

Decide the supported lifecycle first, then add a regression/import-boundary test that reflects that contract.