FIFO Order Allocator

Allocate stock to orders first in, first out: each order consumes the oldest stock first, and the result says which lots it drew from and how much from each.

allocator.receive(lotId, quantity, receivedAt)   // add stock
allocator.allocate(orderId, quantity)            // -> [{ lotId, quantity }, ...] or null
allocator.available()                            // -> total units in stock
allocator.lots()                                 // -> remaining lots in consumption order

Rules

• Orders draw from the oldest lot first (smallest receivedAt); lots with the same receivedAt are consumed in the order they were received.
• A lot may be consumed partially; the remainder stays at the front. Exhausted lots disappear from lots().
• Allocation is all-or-nothing: if stock can't cover the whole order, return null and change nothing.
• allocate returns the lots drawn from, in consumption order, with the quantity taken from each. A quantity of 0 allocates nothing and returns [].
• Lots can arrive out of receivedAt order.