shadowCascadeFramebuffer

Returns the per-cascade shadow framebuffer for the directional sun-shadow pipeline. Each cascade is depth-only: a GL_DEPTH_COMPONENT24 texture holds true hardware depth written by DirectionalDepthProgram's pass (which lets glPolygonOffset apply slope-scaled caster bias). On Kgl.hasShadowSamplers platforms the texture carries LINEAR + COMPARE_REF_TO_TEXTURE and receivers tap it through sampler2DShadow (hardware 2x2 PCF per tap); otherwise receivers read raw depth from the red channel, where GL_NEAREST is mandatory and the software percentage-closer filter does its own bilinear weighting across texels. Clamp-to-edge keeps out-of-footprint UVs from wrapping to the opposite side of the map. Lazily allocated and cached per cascade.