Funnel Drop-off Diagnosis (GA4 + BigQuery)

Goal: diagnose funnel drop-offs for {GOAL_EVENT}. Data: GA4 BigQuery events with event_params. Steps: 1) Construct funnel steps; 2) Use event timestamps to compute step conversion; 3) Segment by device, traffic source, new/returning; 4) Identify friction fields. Output: funnel report with top 5 friction points and experiment ideas.