fix!: preserve repeated TxBuilder.add_data outputs

Store every payload passed to `TxBuilder.add_data` and add each as a separate `OP_RETURN` output (matching upstream `bdk_wallet` behavior) because repeated calls currently overwrite earlier payloads causing finished transactions to silently omit requested outputs.

Start at the TxBuilder.add_data entry point and compare its behavior with upstream bdk_wallet. Verify repeated calls retain every payload and produce separate OP_RETURN outputs in the finished transaction; add or update focused coverage for repeated calls if the surrounding code provides a test location.

Written by the indexing model from the issue text.