fix(memory): persist graph snapshots atomically

[CryoThrust]

Summary

• write memory graph snapshots to a unique temporary file
• atomically replace the live JSONL file with rename
• clean up temporary files on both success and failure
• add regression coverage ensuring persistence leaves no temporary artifact

Closes #4614

Validation

• git diff --check
• Local Vitest execution is blocked because the Homebrew Node 22 binary cannot load libsimdjson.29.dylib; the change is covered by the repository test suite in CI.

[CryoThrust]

Closing because earlier PRs #4656 and #4696 already cover atomic memory graph persistence. Avoiding duplicate implementation.