Final answer:
The foreshadowing detail is Frantz's observation of the little blackboard at the Mayor's office and his sense of foreboding, wondering 'What can it be now?', signaling an impending change in his world.
Step-by-step explanation:
The detail that foreshadows the central idea that Frantz's world is about to change is when Frantz observes the little blackboard on which notices were posted at the Mayor's office and wonders (2) "What can it be now?" This question suggests that the posted notices have historically brought significant and often bad news, indicating that another such impactful event may be imminent. Coupled with the advice from Wachter the blacksmith (4) "Don't hurry so, my boy; you'll get to your school soon enough!" and Frantz's rushed feelings, the foreshadowing is marked by a sense of imminent change that would affect Frantz's routine and expectations.