feat: std/net/kqueue — raw kqueue/kevent bindings, darwin (PLAN-HTTPZ S3)

32-byte darwin struct kevent, EVFILT_READ/WRITE/TIMER, EV_* flags, and
three thin helpers: kev_change (one registration entry), kq_apply
(immediate change, no drain), kq_wait (bounded drain, EINTR reissued,
negative timeout = forever). Off the std.sx barrel by design — the
OS-neutral facade over this and the epoll twin is std.event (S5).
examples/1631 pins zero-cost idle timeout, READ readiness with pending
byte count + udata round-trip, and EV_EOF on peer close; verified under
sx run AND sx build.

examples/expected/1631-net-kqueue.exit

@@ -0,0 +1 @@
+0

examples/expected/1631-net-kqueue.stdout

@@ -0,0 +1,4 @@
+idle wait: timeout, 0 events
+after write: READ ready, 4 pending, udata 7777
+after close: EV_EOF
+kqueue ok