fix: harden strcat overflow and fwrite error handling in model save

src/engine/engine_io.c

@@ -529,7 +529,11 @@ void mj_saveModel(const mjModel* m, const char* filename, void* buffer, int buff
   }
 
   if (fp) {
-    fclose(fp);
+    int write_error = ferror(fp);
+    int close_error = fclose(fp);
+    if (write_error || close_error) {
+      mju_warning("Error saving model to '%s': file may be incomplete or corrupt", filename);
+    }
   }
 }
 

src/ui/ui_main.c

@@ -1894,7 +1894,7 @@ static void shortcuthelp(mjrRect r, int modifier, int shortcut,
   }
 
   // combine
-  strcat(text, key);
+  strncat(text, key, sizeof(text) - strlen(text) - 1);
 
   // make rectangle for shortcut
   int g_textver = SCL(ui->spacing.textver, con);