The lazy instance property uses a GetterCallback that calls the getter and converts the return value through the TypeWrapper. JsArrayBuffer (which extends JsBase<v8::ArrayBuffer>) should convert directly to a v8::Local<v8::Value> since it's a V8 handle wrapper. This is a straightforward conversion.

Let me also verify there are no GC visitability concerns — JsArrayBuffer is a V8 handle wrapper (not persistent), so it should be fine as a return type from a lazy property getter (it's created and returned, not stored).

Let me also check the ExecOutput class for any GC visit concerns with this change. Looking at the header — ExecOutput stores kj::Array<kj::byte> (C++ data), not JS objects. The getters create a new JsArrayBuffer on each call, which is correct for a lazy property (evaluated once, then cached by V8). Since BufferSource was previously returned (which contained a BackingStore that needed visiting), and JsArrayBuffer is a transient V8 local handle, the GC story is actually simpler now — no visitForGc needed for the return value.

This is a clean, straightforward migration. The changes:

1. Remove the copyBytes helper function (no longer needed)
2. Change return types from jsg::BufferSource to jsg::JsArrayBuffer
3. Use jsg::JsArrayBuffer::create(js, data) which allocates and copies in one step

The semantics are preserved (data is copied into a new ArrayBuffer each call). The TypeScript override already declares readonly stdout: ArrayBuffer and readonly stderr: ArrayBuffer, which is consistent with JsArrayBuffer.

LGTM

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