nx_probe_crashstk_live.nx

nx_probe_crashstk_live.nx -- WITNESS: a STACK-OVERFLOW crash under the guard must still print the diagnostic (proves the alternate signal stack works -- without sigaltstack the handler frame cannot be pushed and the process dies silently) and still die by the original signal. The recursion is deliberately NOT a tail call (the add uses the recursive result) so the frames genuinely accumulate. The gate asserts: exit 139 AND stderr contains "the program crashed" (fault address is the guard page -- not assertable exactly, unlike the null probe).