nx_install_hash_test.nx

nx_install_hash_test.nx -- smoke for nx_install_hash SA-7 partial. Exercises: 1. Bad-input gates 2. Tampered plan refused (canary stomp -> NX_HASH_BAD_PLAN) 3. REPRODUCIBILITY gate: two plans with identical content -> same hash 4. SENSITIVITY gates: change any content field -> different hash (probe field, calibration field, policy field, n_selected, total_cost, verdict, variant_id) 5. EXCLUSION gates: change excluded fields (ts_us, canaries- gating-aside) -> same hash 6. nx_install_hash_eq positive + negative SA-7 partial verification gate per NISHI_SELF_ASSEMBLY_ROADMAP.md §8: "Reproducibility gate -- same target + same source corpus + same selector version -> same install_hash across two independent runs on different days". This test proves the deterministic property; cross-day persistence is a future smoke composing nx_fact_log + nx_install_hash.