SQL: Date x Product Matrix (CROSS JOIN)

> This problem is paper-only - see explanation for the canonical SQL solution. You have a small `Products(id, name)` table and a list of dates (e.g. from a `generate_series` or a calendar table). To build a "zero-filled" daily-sales report, you need every (date, product) pair even where there were no sales - the classic CROSS JOIN pattern. Given `dates` (array of ISO date strings, sorted) and `products` (array of `{id, name}`), return the cartesian product as an array of `{date, product_id, product_name}` objects, sorted by date ascending then product_id ascending. **For the auto-grader:** implement `dateProductMatrix(dates, products)`.