TestPrefetchServerRetainVisibility polled the mocked op-status endpoint at
the production _RETAIN_OP_POLL_INTERVAL_S of 0.5s, so each test burned ~1s
of real sleep plus a cross-thread hop to the shared hindsight event loop per
poll — all inside a 5.0s join on the background prefetch thread. On a busy
runner that thread is exactly what gets starved, so it outlived the join and
the assertion saw an empty order list: assert [] == ['recall'].
collapse the interval to 0.01s in these tests (against an AsyncMock there is
no server to be polite to) and route the joins through a helper that asserts
the thread actually finished, so a genuine wedge reports itself instead of
surfacing as a confusing empty-list mismatch.
also drop the two wall-clock assertions that measured runner load rather than
the contract under test: the drain-budget bound goes from 3.0s to budget+10s,
and the eviction test now asserts the status endpoint saw no further calls
(the actual "dropped ops aren't re-polled" contract) instead of timing the
second prefetch at <0.25s.
reproduced by pinning the tests to one core against N spinners on that same
core: at 16x oversubscription pre-fix failed 3/3 and post-fix passed 3/3.
class wall time drops 1.45s -> 0.44s for the recall test.
c0bd7fca68