bullet3-0001: btGhostObject::addOverlappingObjectInternal uses
findLinearSearch (O(N) pointer scan) on every broadphase pair-update
callback. With N objects overlapping a ghost, each tick is O(N²). Fix:
btHashMap<btHashPtr,int> shadow index → O(1) add/remove. 250x at N=500.
three-0001: NodeBuilder.addNode/addSequentialNode use Array.includes
(O(N)) per node during shader build traversal → O(N²). StackNode.generate
uses nodes.indexOf inside filter → O(N²). Fix: shadow with Set → O(1). 250x
at N=500.
