nx_engineer_crash_test.nx

nx_engineer_crash_test.nx -- the Engineer catches a crash vs a clean exit. Runs a known crasher (null write -> SIGSEGV) and a clean control, and proves it distinguishes a SIGNAL DEATH from a normal exit -- the thing the gate runner's exit-code-only check misses. Build lane provides /tmp/eng_crasher.elf and /tmp/eng_clean.elf. Known answer: crasher = CRASH signal 11, clean = exit 0.