stream: drop per-chunk Promise alloc in pipeTo

lib/internal/webstreams/readablestream.js

@@ -111,6 +111,7 @@
   kState,
   kType,
   lazyTransfer,
+  markPromiseAsHandled,
   nonOpCancel,
   nonOpPull,
   nonOpStart,
@@ -1569,9 +1570,18 @@
         }
 
         // Write the chunk - we're already in a separate microtask from enqueue
-        // because we awaited writer[kState].ready.promise above
+        // because we awaited writer[kState].ready.promise above.
+        //
+        // setPromiseHandled's job here is purely to silence the per-chunk
+        // unhandled-rejection event — the chain Promise + noop closure it
+        // would allocate are unobserved by anything else (errors propagate
+        // through writer.closed, which pipeTo monitors via watchErrored).
+        // markPromiseAsHandled sets V8's MarkAsHandled + MarkAsSilent flags
+        // directly, with no per-chunk allocation. The LAST state.currentWrite
+        // is awaited in waitForCurrentWrite during shutdown — markAsHandled
+        // does not affect that subsequent .then chain.
         state.currentWrite = writableStreamDefaultWriterWrite(writer, chunk);
-        setPromiseHandled(state.currentWrite);
+        markPromiseAsHandled(state.currentWrite);
 
         // Check backpressure after each write
         if (dest[kState].backpressure) {
@@ -1675,7 +1685,10 @@
     // "ReadableStreamPipeTo" step 15's "chunk steps".
     queueMicrotask(() => {
       this.state.currentWrite = writableStreamDefaultWriterWrite(this.writer, chunk);
-      setPromiseHandled(this.state.currentWrite);
+      // See comment in pipeTo's fast path on the markPromiseAsHandled vs
+      // setPromiseHandled choice. The per-chunk silencing has no other
+      // observer; errors reach pipeTo via writer.closed monitoring.
+      markPromiseAsHandled(this.state.currentWrite);
       this.promise.resolve(false);
     });
   }

lib/internal/webstreams/util.js

@@ -35,6 +35,7 @@ const {
     kPending,
   },
   getPromiseDetails,
+  markPromiseAsHandled,
 } = internalBinding('util');
 
 const assert = require('internal/assert');
@@ -225,6 +226,7 @@ module.exports = {
   kState,
   kType,
   lazyTransfer,
+  markPromiseAsHandled,
   nonOpCancel,
   nonOpFlush,
   nonOpPull,