The prior assertion `all("turn1" in k or "turn2" in k for k in keys)` was
weak on two counts: it passes vacuously when keys is empty (a regression
that lost all state would slip through), and after turn 2 finalizes only
turn 1 lingers, so it only ever inspected turn 1 anyway. Replace it with an
exact check that one key survives, it is turn 1, and turn 2 never merged
into it — the real isolation invariant the test name claims.
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