When a link element starts a replacement fetch, the old response body
can already have queued its consume-body task. stop_fetch() clears the
fetch algorithms for future work, so that late task re-read an empty
callback and tripped AK::Function's wrapper assertion.
Keep fetch_response_handover() tied to the algorithms object it
selected when creating the consume-body steps. Then make
HTMLLinkElement apply the HTML stale-resource check explicitly with a
fetch generation, so results from superseded link fetches return before
processing the resource.
Add a text test that replaces a stylesheet data URL and waits for the
replacement stylesheet's load event. This covers stale body callbacks
and verifies that superseded stylesheet results do not apply.
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