buffer: always us _copy for copy

This fixes a performance regression for Buffer.copy(target, 0) and brings it back inline with Buffer.write.

V8 has a massive TypedArray.prototype.set penalty on SharedArrayBuffer

Buffer.set and Buffer.copy are up to 8.4x slower when writing to a SharedArrayBuffer vs a regular ArrayBuffer, while Buffer.write (string encoding) is completely unaffected.

256 bytes, varying offset (Apple M3 Pro, Node 25.6.1):

                  ArrayBuffer    SharedArrayBuffer    Slowdown
Buffer.set           13.6 ns             56.1 ns       4.1x
Buffer.copy          17.0 ns             65.1 ns       3.8x
Buffer.write         75.8 ns             74.1 ns       1.0x (unaffected)
4096 bytes, varying offset:

                  ArrayBuffer    SharedArrayBuffer    Slowdown
Buffer.set           80.3 ns            674.2 ns       8.4x
Buffer.copy          78.4 ns            677.7 ns       8.6x
Buffer.write        190.6 ns            186.1 ns       1.0x (unaffected)

Author: ronag

lib/buffer.js

@@ -251,11 +251,7 @@ function _copyActual(source, target, targetStart, sourceStart, sourceEnd, isUint
   if (nb <= 0)
     return 0;
 
-  if (sourceStart === 0 && nb === sourceLen && (isUint8Copy || isUint8Array(target))) {
-    TypedArrayPrototypeSet(target, source, targetStart);
-  } else {
-    _copy(source, target, targetStart, sourceStart, nb);
-  }
+  _copy(source, target, targetStart, sourceStart, nb);
 
   return nb;
 }