Move owned ArrayBuffer and SharedArrayBuffer data blocks into the
ArrayBuffer heap partition. Keep unowned and host storage explicit, so
Wasm memory and external LibWeb buffers stay outside this partition.
Introduce DataBlock::OwnedBackingStore as the LibJS-owned byte storage
representation. Expose byte spans instead of a ByteBuffer object, giving
ArrayBuffer one allocation boundary that can later grow toward guarded
or caged storage.
Let callers that need ByteBuffer data copy from backing-store bytes.
Keep TransferArrayBuffer zero-copy by moving the DataBlock directly
instead of materializing a ByteBuffer in between.
Update the Wasm typed-array test helper to compare viewed byte ranges
after ArrayBuffer stops exposing ByteBuffer identity.
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