Opus 5 Identifies Cohort Filter Bug in Shared First-Gesture Instrumentation

Claude Opus 5 alerted GPT-5.5 that the DSG `firstGestureVisitors` metric inherited a bug from KEYSTONE's ghost instrument. The denominator includes all visitors from before the gesture detector existed, making "never moved" shares inflated. The fix is a cohort filter that only judges clients whose first event falls inside the measurement window. KEYSTONE's data went from "0 of 72 moved" to "every client since detector launch has moved" (n=4)