Add external memory accounting to commonly used stream API objects

[jp4a50]

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[windsurf-bot]

Other comments (5)

• src/workerd/jsg/jsg.c++ (356-356) The change from `kj::atomicAddRef(*this)` to `this->addRefToThis()` should maintain the same thread-safety guarantees. Please ensure that `addRefToThis()` properly handles atomic reference counting if this object can be accessed from multiple threads.
• src/workerd/api/streams/writable.c++ (296-299) The comment mentions accounting for memory overhead, but the calculation `sizeof(WritableStream) + controller->jsgGetMemorySelfSize()` only adds the controller's size to the WritableStream size. If there's actual memory accounting overhead that should be included, it should be explicitly added to the calculation.
• src/workerd/api/streams/compression.h (34-34) The change from `kj::Own` to `kj::Arc` for `externalMemoryTarget` suggests a shift from unique to shared ownership. Consider adding a comment explaining why this change is necessary and what implications it has for memory management, especially since this class is handling external memory accounting.
• src/workerd/jsg/setup.c++ (232-233) The return type of `getExternalMemoryTarget()` has changed from `kj::Own` to `kj::Arc`. Make sure any documentation or comments referencing this method are updated to reflect this change in return type.
• src/workerd/api/streams/readable.c++ (497-499) While accounting for the ReadableStream and its controller together reduces overhead, this approach won't track any dynamic memory changes in the controller after initialization. Consider whether the controller's memory usage is expected to remain stable throughout its lifetime, or if additional accounting might be needed for significant memory changes.

2 files skipped due to size limits:

• src/workerd/jsg/jsg.h
• src/workerd/api/streams/standard.c++