Map regional category leaders

Mastery is composition: control the grain after every stage, join only when needed, calculate at the right level, and shape the final contract deliberately.

Using completed orders, calculate line revenue, join product categories, total revenue by region and category, then return every region-category pair ranked by revenue.

Expected result: Sorted summaries with `region`, `category`, and `revenue`; at least seven stages.

[
  { $match: { status: "completed" } },
  { $unwind: "$items" }
]