Debounce/coalesce contextual user model saves

[tianjianjiang]

Follow-up to #781.

saveContextualUserModel runs after every observed candidate selection. The write is already off the main thread (snapshot via serialize(), atomic write on a serial queue), so there is no UI impact, but a fast typist still produces one full-file write per selection.

Coalesce saves: e.g. mark dirty and flush after a short idle interval, on deactivation, and at termination. Must not lose the final state on abnormal exit more than the current behavior does.