Return transient context from the selected cache loader

Can get_or_fetch return optional winning-loader context on success or error, so concurrent queries can trace which load they waited for? For hybrid fetches, the context follows the loader that runs, including a later required loader, while ready cache hits carry no past origin. What is the smallest compatible API that reuses one fetch path, without another map, a sweeper, or context in cached values?

I used AI to help draft this.