nx_x86_64_test.nx

nx_x86_64_test.nx -- foundation smoke for the x86_64 emit library. Hand-sequences nx_x86_64 primitives to build a write+exit program asm file, prints it to stdout. The companion bash wrapper then assembles + links + runs the produced binary natively on x86_64. Two layers verified by this smoke: 1. Each emit primitive produces the expected asm text (this file). 2. The asm assembles + runs natively (bench/nx_x86_64_smoke.sh). The hand-built asm is the canonical "hello world" + sys_exit(0).