The prewarm registered its cancel event in _active_generate_cancel, but a
begin_generate arriving mid-warmup overwrote that slot with its own event
and then queued its worker behind the full warmup on _generate_lock. From
that point unload and cancel_generate signalled the wrong event, so the
warmup could no longer be aborted and the first real request waited out
the 9-54s the prewarm exists to hide.
Track the prewarm's event in a dedicated _prewarm_cancel slot (cleared
identity-checked alongside _active_generate_cancel) and signal it from
begin_generate before registering the real job's event, and from direct
generate() calls that skip begin_generate. The warmup then aborts at its
next step boundary and the real job takes the lock, while unload/cancel
keep working against whichever run is actually active.
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