Variable GPU_LAYOUT_NODE_WORDConst

Word positions inside a node, matching the tracer's WGSL BvhNode:

leftMin: vec3f,  leftIndex: u32,
leftMax: vec3f,  rightIndex: u32,
rightMin: vec3f, leftCount: u32,
rightMax: vec3f, rightCount: u32,

A child with count > 0 is a leaf holding count primitives starting at index; count == 0 is an inner node at index. Child side (0 left, 1 right) keeps its box at words 8 * side (min) and 8 * side + 4 (max).