- NodeToElementsMap: Inverse connectivity (node → elements touching it)
- ElementNodeInfo: Tracks element ID and local node index
- get_node_spider() finds all nodes coupling with given node
- NodalStiffnessContribution: Storage for 3×3 blocks per node
- matrix_vector_product_nodal() computes K_i*u at single node
- print_spider_info() debugging diagnostics
- 234 lines: Infrastructure for node-by-node assembly