nx_select_joint_test.nx

nx_select_joint_test.nx -- smoke for nx_select_joint SA-5 MVP. Demonstrates the LOAD-BEARING joint-DAG property: independent argmin picks a locally-optimal pair that is layout-INCOMPATIBLE; joint solver picks a globally-optimal pair that costs more locally but composes through the edge. Two-node DAG: [node A: variant_class=100] --edge--> [node B: variant_class=200] Producer candidates (node A): A_cheap_row : cost=100, out_layout=ROW_MAJOR A_medium_col : cost=300, out_layout=COL_MAJOR A_expensive_col: cost=900, out_layout=COL_MAJOR Consumer candidates (node B): B_cheap_col : cost=100, in_layout=COL_MAJOR B_expensive_row : cost=900, in_layout=ROW_MAJOR Per-node argmin (single-class): node A picks A_cheap_row (cost=100) node B picks B_cheap_col (cost=100) But the edge A_cheap_row.out=ROW vs B_cheap_col.in=COL is INCOMPATIBLE. Joint argmin: (A_cheap_row, B_expensive_row): row->row compatible; total=100+900=1000 (A_medium_col, B_cheap_col): col->col compatible; total=300+100=400 <- WIN (A_expensive_col, B_cheap_col): col->col compatible; total=900+100=1000 The joint solver must pick (A_medium_col, B_cheap_col) -- INDICES (1, 0) -- demonstrating that the locally-suboptimal A_medium_col is the globally-optimal choice given the edge constraint. SA-5 verification gate per NISHI_SELF_ASSEMBLY_ROADMAP.md §8: "DAG composition smoke -- same algorithm, two layout choices, joint solver picks bit-coherent layouts across nodes"