Build regional running revenue

Window functions calculate across related documents without collapsing them into one group. Partitioning creates independent regional streams; sorting defines sequence; the window frame decides which preceding values contribute to each result.

For completed orders, number each order within its region by date and calculate running regional revenue. Return `orderId`, `region`, `placedAt`, `regionalOrderNumber`, and `runningRevenue`, ordered by region and date.

Expected result: Nine chronological order records that retain document-level detail while carrying regional running totals.

[
  { $match: { status: "completed" } },
  { $set: { placedDate: { $dateFromString: { dateString: "$placedAt" } } } }
]