Save demonstration files atomically

[eust-w]

Summary

• write demonstration safetensors to a unique temporary file in the destination directory
• fsync the completed file before atomically replacing the destination
• fsync the parent directory where supported
• clean up temporary files after both successful and failed saves
• add regression coverage for failure preservation and successful replacement

Why

Demo.save currently writes directly to the final path. If serialization is interrupted, the process crashes, or the filesystem returns an error, an existing demonstration can be truncated or replaced by a partial file.

Saving in the same directory and using os.replace keeps the previous demonstration intact until a complete safetensors file is ready. This is especially useful for long visual demonstrations, where recollection is expensive.

Compatibility

There is no public API or safetensors schema change. DemoRecorder.save_demo continues to return the same final path, and errors still propagate to the caller.

Validation

• regression test: simulated partial write raises while the previous demo remains byte-for-byte unchanged
• regression test: successful save atomically replaces the previous file and removes the temporary file
• real safetensors write/read round-trip passed with a temporary filename
• Black 23.3.0, Ruff 0.0.272, Python compilation and git diff checks passed