nx_select_test.nx

nx_select_test.nx -- smoke for nx_select SA-4 MVP. Exercises six scenarios across the selection pipeline: 1. Empty input -> NX_SEL_BAD_INPUT 2. Single candidate that fits -> returns that index 3. Two candidates, one's ISA mask excludes the host -> picks the fit 4. Two candidates both fit, different cost -> picks lower cost 5. Policy weight swap (throughput vs energy) flips selection 6. Tier filter (variant requires HPC, host is LAPTOP) -> filtered out SA-4 verification gate per NISHI_SELF_ASSEMBLY_ROADMAP.md §8: "picks variants A on hardware H_A and B on H_B with both smokes passing on their respective hosts" -- demonstrated in code by synthesizing probes for both hosts and asserting different selections.