feat: add warning in DOOM for missing wad

programs/src/doom/doom_wad_check.cpp

@@ -0,0 +1,22 @@
+/*
+    * doom_wad_check.cpp
+    * Warns the user and exits cleanly if no DOOM WAD is present
+    * Copyright (c) 2026 Daniel Hammer
+*/
+
+#include <gui/dialogs.hpp>
+#include <montauk/syscall.h>
+#include <stdio.h>
+
+extern "C" [[noreturn]] void doom_warn_missing_wad(const char* path) {
+    char message[400];
+    snprintf(message, sizeof(message),
+        "No DOOM WAD file was found at %s.\n\n"
+        "MontaukOS cannot ship proprietary DOOM WADs (doom1.wad, doom.wad, "
+        "freedoom1.wad, etc.) on its ISO images due to copyright restrictions.\n\n"
+        "Copy a legally obtained WAD file to that path and relaunch DOOM.",
+        path);
+
+    gui::dialogs::message_box("DOOM", message, gui::dialogs::MESSAGE_BOX_OK);
+    montauk::exit(1);
+}