feat(wasm-utxo): add helper to create OP_RETURN output scripts

Add utility function that creates OP_RETURN output scripts with optional
data payload. This supports both empty OP_RETURN scripts and ones with
custom data.

Issue: BTC-2980

Co-authored-by: llm-git <llm-git@ttll.de>

Author: OttoAllmendinger

packages/wasm-utxo/js/fixedScriptWallet/index.ts

@@ -61,3 +61,23 @@ import type { ScriptType } from "./scriptType.js";
 export function supportsScriptType(coin: CoinName, scriptType: ScriptType): boolean {
   return FixedScriptWalletNamespace.supports_script_type(coin, scriptType);
 }
+
+/**
+ * Create an OP_RETURN output script with optional data
+ *
+ * @param data - Optional data bytes to include in the OP_RETURN script
+ * @returns The OP_RETURN script as a Uint8Array
+ *
+ * @example
+ * ```typescript
+ * // Empty OP_RETURN
+ * const script = createOpReturnScript();
+ *
+ * // OP_RETURN with data
+ * const data = new Uint8Array([1, 2, 3, 4]);
+ * const script = createOpReturnScript(data);
+ * ```
+ */
+export function createOpReturnScript(data?: Uint8Array): Uint8Array {
+  return FixedScriptWalletNamespace.create_op_return_script(data);
+}