nx_install_attest_test.nx

nx_install_attest_test.nx -- smoke for nx_install_attest helpers. Exercises: 1. Hex encode produces 64 ASCII lowercase chars for a known digest 2. Hex encode/decode roundtrip on random digest 3. Hex decode tolerates both lowercase a-f AND uppercase A-F 4. Hex decode rejects invalid chars (space, 'g', '-', etc.) 5. Bad-input gates (null in / null out for both encode + decode) 6. verify_against on matching digest -> VERIFIED 7. verify_against on mismatched digest -> TAMPERED 8. verify_against on null plan / canary-stomped plan -> BAD_INPUT 9. verify_against integration with the pipeline: pipeline -> hash A; later verify_against(plan, A) -> VERIFIED 10. Sealed-enum gate