Prevent spreadsheet formula execution in exports

Inputs: [EXPORT CODE], [FIELD LIST], [TARGET SPREADSHEET]. Find user-controlled fields that may be interpreted as formulas. Propose encoding and tests for leading formula characters while preserving round-trip expectations. Output: A threat table and local regression tests. Check: Specify how to open a synthetic export safely in the target spreadsheet and verify no formula executes while values remain intelligible; do not claim this passed unless actually tested. Mark missing details for review.