After a post-pandemic boom in luxury sales, it’s no longer a question of whether a downturn is coming, but how deep, and how long it will be. Sector bellwether LVMH’s soft sales report last week (9 percent growth in the third quarter for the critical fashion and leather goods unit that included Louis Vuitton and Dior, or about half its pace in the first half of the year) proved that not even the biggest brands were immune to economic…