fmodata: investigate tsdown warning about invalid define option in build

[eluce2]

During pnpm --filter @proofkit/fmodata build on 2026-03-15, tsdown emitted:\n\nWarning: Invalid input options (1 issue found) - For the "define". Invalid key: Expected never but received "define".\n\nThe build still succeeds, but this likely indicates stale or unsupported tsdown config and could become a future break.\n\nAcceptance criteria:\n- Reproduce warning locally\n- Identify source of define option in fmodata tsdown/rolldown config\n- Update config to remove warning while preserving build output\n- Ensure pnpm --filter @proofkit/fmodata build runs warning-free