The claim that energy produced by nuclear fusion comes from mass defect via mass-energy equivalence is a matter of established scientific fact and everyday observation in nuclear physics, requiring no citation.
The claim is a fundamental, textbook physical principle governing nuclear reactions (mass-energy equivalence, E=mc^2). None of the retrieved papers are direct nuclear physics studies on fusion mass defect; they are irrelevant biomedical or unrelated physics preprints. Since the claim is basic common knowledge in physics, no citation is necessary and Step 0 classifies it as COMMON_KNOWLEDGE.