Skybox: replace sphere-mesh approach with a fullscreen triangle that
reconstructs the world-space ray direction analytically via inverse
projection/view matrices. Eliminates clip.w=0 degeneracy at the horizon
and correctly maps altitude to dir.z in the Z-up coordinate system.
Clouds: hemisphere mesh was storing altitude in aPos.y (Y-up convention);
the Z-up view matrix projected this sideways, making clouds appear
vertical. Store altitude in aPos.z and update the fragment shader to
read dir.z as altitude and dir.xy as the horizontal UV plane.
