OUTER slips back into our headphones with “Svartsengi,” a second glimpse of the forthcoming Glowing Mountains in the Sky , and it’s a return that feels both intimate and tectonic. The Belgian composer-producer once again navigates the borderlands between contemporary classical, ambient post-rock and shadowy chamber pop, but this time he lets the edges smudge a little more. The result lands somewhere between a whispered confession and a widescreen landscape—fans of Sigur Rós,