TestDialecticLifecycleSmoke._await_thread did a single join(timeout=3.0) and
then proceeded regardless of whether the background dialectic thread had
finished. On a loaded CI runner (6 parallel test slices) the prewarm thread's
completion can slip past that 3s window, so the join times out silently and the
test reads _prefetch_result before the worker wrote it — the intermittent
'session-start prewarm must land in _prefetch_result' failure.
Join in a loop up to a 30s ceiling and assert the thread is actually dead, so a
genuine hang surfaces as a clear failure instead of a timing race. Reproduced
the old failure deterministically (5/5 fails with a 3.5s prewarm delay) and
confirmed the fix (0/8) before/after.
bb1c8b6f1a