fix: clarify chapter 8 hallucination safeguards

[voidborne-d]

Summary

• clarify that the Chapter 8 Matterport example combines two hallucination safeguards: evidence gathering and an explicit information-sufficiency check
• update the Prospectus exercise text to match that teaching point and fix the heading to Exercise 8.2
• add a small validation script so the three Chapter 8 notebook variants stay in sync

Testing

• python3 scripts/check_hallucination_lesson_wording.py