feature: add debounced autosave with flush on navigation, blur, and shutdown

[chaoxu]

Summary

Add debounced autosave for file-backed documents, with explicit flush points beyond the idle timer.

Scope

• Save after a configurable idle delay instead of requiring manual save for every change.
• Flush pending changes on:
  • file switch
  • window/app blur or backgrounding
  • explicit close
  • shutdown
• Keep one debounce timer per open document.
• Do not let repeated edits queue overlapping stale writes.

Why

Live-save only feels safe when it is predictable:

• typing should not block on every keystroke
• switching away from a file should not strand recent edits in memory
• shutdown should not drop the last few seconds of work

This is the minimum user-visible behavior needed for a viable autosave model.

Notes

This issue is only the trigger/timing layer. It depends on a safe write pipeline and should not try to solve conflict handling or crash recovery by itself.