Function sceneEffectiveLineLoad

Fold a scene graph into the effective line load that can be drawn concurrently. Plain groups and partitions sum their children; LOD groups take the maximum because their levels are substitutive. Two levels can overlap briefly during a cross-fade; accepting that at-most-2× transient undercount keeps the frozen default biased toward capsule quality. The max also treats every independent ladder as if it reached its finest level at once, which can over-count a normally framed scene and choose the lower- quality quad. Using each ladder's default_level instead was rejected because the decision must remain safe after view-driven level changes. A lines node is a leaf for this purpose: additive line ladders already advertise their summed total in the parent node's authored n_segments.