nx_select_joint_chain_test.nx

nx_select_joint_chain_test.nx -- smoke for nx_select_joint_chain SA-5 N-node linear chain selector. Exercises: 1. Degenerate N=1 chain falls back to single-class selection 2. N=0 -> BAD_INPUT 3. N=3 linear chain A -> B -> C demonstrates the LOAD-BEARING property: middle node forced into non-locally-optimal variant so the chain composes 4. N=4 linear chain A -> B -> C -> D where the optimal pick at each interior node depends on BOTH neighbors 5. No-feasible-chain detection (incompatible layouts in any pair) SA-5 remainder 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".