A primary market shift to adjustable-rate mortgages and a decline in fixed-rate prime mortgages is continuing to take its toll on the supply of agency MBS in the capital markets. And that has done little to cool investor demand for MBS product or soften pricing. According to numbers compiled by Inside MBS & ABS, the volume of total mortgage securities outstanding rose by an anemic 0.4 percent in the third quarter to reach $4.221 trillion.…