Function worker

port/coroutine_test.cpp:32–38

    do { if (!(cond)) { fprintf(stderr, "FAIL %s:%d %s\n", __FILE__, __LINE__, #cond); std::exit(1); } } while (0)

void worker(void *arg) {
    int id = (int)(intptr_t)arg;
    for (int i = 0; i < 3; i++) {
        g_seq[g_seq_n++] = id * 10 + i;
        port_coroutine_yield();
    }
}

/* Round-robin alternation: two coroutines, three yields each. The
 * expected interleave is a0,b0,a1,b1,a2,b2 because the main loop