Create a basket snapshot

Array expressions can inspect a collection without changing the document grain. `$size` counts elements, while `$arrayElemAt` reads one position using a zero-based index.

Return `orderId`, `lineCount`, and `firstProductId` for every order. Count `items`, then read the first product ID without unwinding.

Expected result: Twelve compact basket summaries in the original order.

[
  { $project: { _id: 0, orderId: "$_id" } }
]