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.

tests/test_config.c

@@ -1672,7 +1672,7 @@ static void test_config_receive_rejects_invalid_iconv_spec() {
 static void test_config_super_mode_wire_roundtrip() {
   if (is_running_under_valgrind())
     return;
-  int modes[] = {SUPER_MODE_AUTO, SUPER_MODE_ON, SUPER_MODE_OFF};
+  SuperMode modes[] = {SUPER_MODE_AUTO, SUPER_MODE_ON, SUPER_MODE_OFF};
   for (size_t i = 0; i < sizeof(modes) / sizeof(modes[0]); i++) {
     int p[2];
     EXPECT_EQ_INT(socketpair(AF_UNIX, SOCK_STREAM, 0, p), 0);