scaffold: 4 new module dirs + registry/Makefile wiring (stubs)

Pre-scaffolding for the next batch (CVE-2023-32233, CVE-2023-4622,
CVE-2022-25636, CVE-2023-0179). Each module ships as a 21-line
stub returning PRECOND_FAIL; parallel agents fill in the real
detect/exploit/--full-chain implementations.

This commit keeps registry.h / iamroot.c / Makefile in one place
so the 4 parallel agents don't collide on shared-file edits — they
each own a single iamroot_modules.c.

Build clean on Debian 6.12.86; --list shows all 24 modules
including the 4 new stubs.

modules/af_unix_gc_cve_2023_4622/iamroot_modules.h

@@ -0,0 +1,12 @@
+/*
+ * af_unix_gc_cve_2023_4622 — IAMROOT module registry hook
+ */
+
+#ifndef AF_UNIX_GC_IAMROOT_MODULES_H
+#define AF_UNIX_GC_IAMROOT_MODULES_H
+
+#include "../../core/module.h"
+
+extern const struct iamroot_module af_unix_gc_module;
+
+#endif