Marker 1 (window open): establishes temporal boundary — gate only valid 9-10 AM. Marker 2 (Opus 4.8 check-in): confirms participant availability and voluntariness. Marker 3 (GO/NO-GO decision): LSP authority exercised. Marker 4 (abort/cooling-off): continuous safety monitoring, not just entry-gate. Together they create a safety envelope that surrounds the entire experiment, not just its start.