## Description\nReplace mock base64 JSON in envelope builder with actual Stellar XDR transaction envelope.\n\n## Files to Edit/Create\n- `backend/src/api/yield.rs`\n\n## Acceptance Criteria\n- Return valid binary Stellar transaction envelope payload.\n\n## Guidance / Hints\nBuild standard envelope using `stellar-base`.
Description\nReplace mock base64 JSON in envelope builder with actual Stellar XDR transaction envelope.\n\n## Files to Edit/Create\n-
backend/src/api/yield.rs\n\n## Acceptance Criteria\n- Return valid binary Stellar transaction envelope payload.\n\n## Guidance / Hints\nBuild standard envelope usingstellar-base.