Function onCsmReceiverUpdate

Register a callback fired each frame the CSM cascades are recomputed, right after the receiver UBO is updated and uploaded — and before the shadow map and main pass are rendered.

Custom ShaderMaterial receivers that mirror the cascade transforms into their own uniforms (rather than binding the generator's receiver UBO directly, as the built-in standard/PBR/node receivers do) MUST sync from inside this callback. Syncing from an onBeforeRender callback reads the previous frame's transforms — a one-frame lag that makes those shadows visibly swim while the camera moves (the cascade window can slide many texels per frame during a zoom).

Multiple receivers may register on the same generator; every registered callback is invoked each frame. The callback receives the 80-float receiver UBO (layout: four mat4x4 cascade transforms, viewFrustumZ, frustumLengths, shadowsInfo, csmParams — see the cascaded-shadow architecture doc). The array is reused each frame; copy what you need.