Build an inventory management system

• Products (SKU, name, reorder threshold) and warehouses; stock is tracked per product per warehouse as onHand and reserved, with available = onHand - reserved
• receive(sku, warehouseId, qty) and ship(sku, warehouseId, qty) change onHand; stock can never go negative, and shipping more than available is rejected
• reserve(orderId, items) holds stock for an order across one or more warehouses all-or-nothing; release(orderId) returns it, and commit(orderId) converts it to a shipment
• Reservations expire after a configurable time, using an injectable clock, and expired holds free the stock
• When available stock falls at or below the threshold, a low-stock alert is emitted once (until restocked), through an observer or event emitter
• Concurrent updates to the same SKU are applied one at a time, even if the code awaits between steps (per-key mutex or serial queue), and the tests demonstrate it

Only after every requirement works.

• Transfers between warehouses as an atomic pair of operations
• Audit log of every stock movement with before and after values
• Allocation strategies (nearest or fullest warehouse) as pluggable policies