A new mixed-reality headset developed by Samsung Electronics Co. and Alphabet Inc.’s Google has hit the market for about half the price of Apple Inc.’s $3,499 Vision Pro — the product it mostly closely resembles and will directly compete against.The $1,800 device, called the Galaxy XR, marks a bold attempt to draw interest around a product category that has so far failed to resonate with ordinary consumers. The companies first teased the gadget last December, referring to it by a codename: “Proj...