pickFramebuffer

Pick-pass FBO: RGBA8 color (pick IDs) + DEPTH24 (DEPTH16 on WebGL1 / GLES2 fallback). Allocate via ensurePickFramebuffer. The depth precision controls how far from the camera a non-terrain pick can land before depth-readback drift recovers the surface point on the far clipping plane (Earth-scale view: antipode / under-ground artefacts).