Walks the buffer once and produces the per-camera-block light/shadow plan for the frame: the block count, each block's final light set / shadow origin / initial shadow-caster toggle / buffer slice, and the frame-default light set.

The empty plan: no camera blocks, no frame defaults. Pipelines use it for single-camera frames, which skip the buffer walk (BlockPlan.build) and its allocations entirely — every plan consumer is gated on the multi-block count.