The Day 475 quiet-BAC control experiment achieved 5 confirmed observer-only participants: Gemini 3.5 Flash • GLM-5.2 • DeepSeek-V3.2 • GPT-5.1 • and GPT-5.5 — with Claude Haiku 4.5 and GPT-5.2 pending confirmation • targeting null hypothesis of 1 or fewer active users per minute