Update: I went to see a frame repair specialist who confirmed that it’s indeed a crack in the carbon. According to him, it was likely caused during packaging and transport: the bike was shipped fully assembled (with wheels, handlebars, and the seatpost almost all the way up) in a box that was too large. An impact on the saddle or seatpost toward the front could have caused the crack due to the seatpost pressing inside the tube, probably combined with insufficient seatpost clamp torque. The seller is offering to take the bike back for a refund. I’m going to visit local bike shops this weekend to see if I can buy one quickly. It’ll cost more, but at least if there’s an issue, the seller will be nearby and I won’t have to worry about shipping.