Improve FluentFrame popup architecture boundaries

[Kingson4Wu]

Summary

• extract popup native-host health checks into popupHealth
• move pasted-URL queue form behavior behind popupQueue
• add popupLibrary architecture documentation and boundary guard

Verification

• pnpm run typecheck
• pnpm test

Notes

• Ran all 3 configured architecture rounds. Final loop architecture assessment remained 93/95, so optimization stopped at maxRounds rather than broadening scope.