Group Inventory Items by Warehouse for WMS Report

Transform a flat array of inventory line items — each carrying a warehouse code, SKU, and quantity — into a grouped summary object keyed by warehouse, with total quantity aggregated per location. You will practice `groupBy` for structural reshaping and `reduce` for numeric aggregation within each group. This pattern appears wherever a target system (WMS, ERP, or BI tool) requires data pre-organized by a dimension before it can ingest or render it correctly.