perf(rpc): move the result subtree out of the parse tree

parseRpcResult copied the "result" node out of the parsed reply,
because indexing the JSON object yields an lvalue reference. On large
replies - full transaction lists, unspent-note sets - that duplicated
the entire subtree for no reason, and it also meant a secret-bearing
result then existed in two trees at once.

- The result node is now moved out of the parse tree instead of
  copied, so the secret lives only in the returned value that the
  calling code already wipes. The leftover result slot is moved-from
  (null), so the existing secret-scrub of the discarded tree still
  runs harmlessly.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/rpc/rpc_client.cpp

@@ -451,8 +451,8 @@ json RPCClient::parseRpcResult(long httpCode, const std::string& body, bool scru
         throw RpcError(errCode, "RPC error: " + err_msg, httpCode);
     }
 
-    json result = response["result"];   // a COPY of the subobject (operator[] yields an lvalue ref)
-    if (scrubSource) scrubJsonSecrets(response);  // zero the discarded tree's secret before it frees (B7)
+    json result = std::move(response["result"]);  // move the subtree out: the secret now lives only here
+    if (scrubSource) scrubJsonSecrets(response);   // response["result"] is now moved-from (null); harmless (B7)
     return result;
 }