benchmark: respect stream/iter broadcast backpressure

Only decrement the remaining byte count after a stream/iter broadcast
write is accepted. If writeSync() is blocked by strict backpressure,
fall back to the async write() path for the same chunk.

Signed-off-by: Kamat, Trivikram <16024985+trivikr@users.noreply.github.com>
Assisted-by: openai:gpt-5.5
PR-URL: #63314
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>

Author: trivikr

benchmark/streams/iter-throughput-broadcast.js

@@ -128,9 +128,11 @@ function benchIter(chunk, numConsumers, datasize, n, totalOps) {
     let remaining = datasize;
     while (remaining > 0) {
       const size = Math.min(remaining, chunk.length);
-      remaining -= size;
       const buf = size === chunk.length ? chunk : chunk.subarray(0, size);
-      writer.writeSync(buf);
+      if (!writer.writeSync(buf)) {
+        await writer.write(buf);
+      }
+      remaining -= size;
     }
     writer.endSync();