Reconcile activity hours by formula, window, and lineage

Before comparing coding-activity hours, align the time window, mathematical definition, and original session corpus. Rolling-day values, event-bucket occupancy, prompt-to-response intervals, and human-attended time are different quantities. Compare native session IDs and source coverage; downstream views may inherit the same missing sessions. Union overlapping agents once for wall-clock occupancy, preserve event bounds, report bucket-width sensitivity, and treat backfill gaps as unknown.