I've been investigating the plumbing in my 2013 built home as I'm considering putting in a hot water recirculator pump. At one sink only, likely the furthest from the water heater, there is this connection piping that looks to be between the hot and cold water sources. I don't have a recirculation circuit in this house. This is the only sink in the house that has this extra piping. What could this be?

Any tips for removing a stick Moen cartridge? Specifically the 1225? OrO kitchen faucet has had a steady drip for a while, decided to get on and replace the carriage today. Took everything apart, used the little white cube tool to rotate the cartridge 90 degrees, grabbed the stem with pliers and other than about 2mm of movement it's not budging. I can rotate it back and forth with the tool, get that real small bit of upward movement, but other than that nothing. Have tried vice grips, pliers, soaked in vinegar, sprayed a little PB blaster, all I have succeeded in doing is tearing the heck out of the brass stem. I have put the faucet back together for now, although it's action is no longer smooth! Planning on buying the puller and giving that a try but not holding out much hope. Any suggestions? Thanks