fix: avoid temp-file collisions during concurrent spreadsheet saves

[interpreterwork]

Summary

• generate unique temp artifact names per /api/save request (temp_changes_<uuid>.bin, params_save_<uuid>.xml)
• ensure the hash-specific output directory exists before writing temp artifacts
• keep cleanup scoped to each request's temp files to avoid cross-request deletion

Why

The save endpoint reused shared temp filenames, so overlapping autosaves could delete each other's temp files and return conversion failures.