A closed fd 1 is a fd 1 waiting to be handed to something else

The merged exit closes stdout so the compiler reads EOF and learns the program
has finished. POSIX then hands descriptor 1 to the next thing that asks — a
socket, a module's object file — and the llc after that inherits it as its
stdout. /dev/null takes the slot back, and the EOF is unaffected because the
pipe's write end is genuinely gone.

Verified beyond the headless suite: sand.flan builds and runs merged under
Xvfb, one process with no children, and a game-draw typed in at the socket is
drawing frames a second later.

lib/dev.ml

@@ -1888,6 +1888,7 @@ let merged_main_source = {c|
 #include <caml/callback.h>
 #include <caml/mlvalues.h>
 #include <pthread.h>
+#include <fcntl.h>
 #include <stdatomic.h>
 #include <stdint.h>
 #include <stdio.h>
@@ -1918,6 +1919,11 @@ static void flan_merged_exit(int32_t status) {
   (void)status;
   fflush(NULL);
   close(1);
+  /* ...and take fd 1 straight back, because POSIX hands out the lowest free
+   * descriptor: leave it open and the compiler thread's next socket or file
+   * becomes this process's stdout, and the next llc inherits it. The EOF is
+   * unaffected — the pipe's write end is genuinely gone. */
+  if (open("/dev/null", O_WRONLY) < 0) { /* nothing useful to do about it */ }
   for (;;) pause();
 }