[Code0x58]

Saving now creates an exclusive temporary file beside the destination, then atomically replaces it and removes the temporary file on failure. Existing backup files and symlinks are untouched, overlapping saves use independent files, and pathlib paths work through save as well as load.

Validation: 29 tests pass, including backup/symlink safety, interleaved saves, failed-replacement cleanup, and pathlib paths.

Stack position 4 of 9. Depends on #16; this PR targets codex/persist-deletions so its diff contains only this fix. Addresses review finding 5.

Stack overview: #14. Next: #18.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.63%. Comparing base (1d4ff24) to head (bd84f30).
⚠️ Report is 6 commits behind head on master.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #17      +/-   ##
==========================================
+ Coverage   95.89%   98.63%   +2.74%     
==========================================
  Files           1        1              
  Lines         146      147       +1     
==========================================
+ Hits          140      145       +5     
+ Misses          6        2       -4     

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