fix fresh-install build: add preflight script, restore third_party binaries, remove WebEngine dep

- Add build_preflight.sh to create obj/ dirs that git can't track (body/board/obj, panda/board/obj)
- Wire preflight into build_only.sh and launch_chffrplus.sh
- Restore missing third_party binaries (libyuv, snpe, acados, maplibre) that were text pointers
- Remove dead Qt5WebEngineWidgets dependency from UI SConscript

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

third_party/libyuv/x86_64/include/libyuv.h

@@ -0,0 +1,32 @@
+/*
+ *  Copyright 2011 The LibYuv Project Authors. All rights reserved.
+ *
+ *  Use of this source code is governed by a BSD-style license
+ *  that can be found in the LICENSE file in the root of the source
+ *  tree. An additional intellectual property rights grant can be found
+ *  in the file PATENTS. All contributing project authors may
+ *  be found in the AUTHORS file in the root of the source tree.
+ */
+
+#ifndef INCLUDE_LIBYUV_H_
+#define INCLUDE_LIBYUV_H_
+
+#include "libyuv/basic_types.h"
+#include "libyuv/compare.h"
+#include "libyuv/convert.h"
+#include "libyuv/convert_argb.h"
+#include "libyuv/convert_from.h"
+#include "libyuv/convert_from_argb.h"
+#include "libyuv/cpu_id.h"
+#include "libyuv/mjpeg_decoder.h"
+#include "libyuv/planar_functions.h"
+#include "libyuv/rotate.h"
+#include "libyuv/rotate_argb.h"
+#include "libyuv/row.h"
+#include "libyuv/scale.h"
+#include "libyuv/scale_argb.h"
+#include "libyuv/scale_row.h"
+#include "libyuv/version.h"
+#include "libyuv/video_common.h"
+
+#endif  // INCLUDE_LIBYUV_H_