refactor(config): replace SUPER_MODE_* macros with SuperMode enum

Type Config.super_mode as SuperMode (a proper C enum) instead of a bare
int.  The wire boundary still carries the mode as an int: send casts the
enum explicitly and receive reads a temporary int, validates the
AUTO..OFF range, then casts.  Emitted bytes and accepted values are
unchanged.  ModuleGateContext.super_mode_override keeps its -1 sentinel
as int with an explicit cast at the apply site.

Behavior preserved.

src/shared/config.c

@@ -1293,14 +1293,14 @@ static bool receive_iconv_spec(int fd, Config* c) {
  * validated to the SUPER_MODE_AUTO..SUPER_MODE_OFF range (also re-checked by
  * validate_received_config). */
 static bool send_privilege_options(int fd, const Config* c) {
-  return send_int(fd, c->super_mode);
+  return send_int(fd, (int)c->super_mode);
 }
 
 static bool receive_privilege_options(int fd, Config* c) {
   int mode;
   if (!receive_int(fd, &mode) || mode < SUPER_MODE_AUTO || mode > SUPER_MODE_OFF)
     return false;
-  c->super_mode = mode;
+  c->super_mode = (SuperMode)mode;
   return true;
 }