nx_sigpipe_gate.nx

nx_sigpipe_gate.nx -- proves the SIGPIPE root fix, and proves the tooth can SEE the disease it cures (debt seq1261/seq1126 family). THE FLAW: a process that writes to a peer which has closed its end takes SIGPIPE's default action -- TERMINATE. For a daemon that is an outage with no diagnosis: it dies holding a healthy listening socket, the supervisor reports a crash-loop, and the request that triggered it looks failed even when the work completed. Every long-running server in the tree ran this way (a tree-wide grep for SIGPIPE handling returned NOTHING before this gate). THE PAIR (why this gate is not vacuous): T1 forks a child that does NOT install the ignore and confirms the kernel KILLS it with signal 13 -- if this ever goes green-by-accident the tooth has stopped measuring anything. T2 forks a child that DOES install it and confirms the same write instead returns -EPIPE and the child exits 0. One tooth shows the disease, the other shows the cure, and neither can pass for the other's reason.