feature: add hot-exit backups and crash recovery separate from normal saves

[chaoxu]

Summary

Add crash-recovery / hot-exit backups that are separate from normal file saves.

Scope

• Keep recovery snapshots outside the workspace.
• Create them on a debounced schedule while content is dirty.
• Restore from backup after crash / force quit / failed shutdown.
• Delete stale backups after successful save or explicit discard.
• Make this distinct from autosave-to-disk.

Why

Autosave reduces loss from normal editing, but it does not replace recovery.
Separate backup snapshots protect against:

• app crash
• OS restart / forced shutdown
• save failure
• partial writes

This is one of the main reasons live-save can feel safe in mature editors.