app: migrate all engine access to the C ABI facade (nm U _ZN5olive = 0)

Every app module now reaches liboakengine exclusively through
oakengine_* C calls, EngineEventBridge subscriptions and app-side
handle headers (cliphandle/keyframehandle/nodevaluehandle/oakvaluehelper).
Direct C++ command construction, engine signal connect()s, and engine
type usage in MOC-visible signatures are gone: 557 -> 0 undefined
olive:: symbols in oak-editor.

app/widget/viewer/vieweroutpututils.cpp

@@ -0,0 +1,76 @@
+/***
+
+  Oak - Non-Linear Video Editor
+  Copyright (C) 2026 Oak Team
+
+  This program is free software: you can redistribute it and/or modify
+  it under the terms of the GNU General Public License as published by
+  the Free Software Foundation, either version 3 of the License, or
+  (at your option) any later version.
+
+  This program is distributed in the hope that it will be useful,
+  but WITHOUT ANY WARRANTY; without even the implied warranty of
+  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+  GNU General Public License for more details.
+
+  You should have received a copy of the GNU General Public License
+  along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+***/
+
+#include "vieweroutpututils.h"
+
+namespace olive {
+
+VideoParams video_params_from_pod(const oak_video_params &pod)
+{
+	void *vp = oakengine_video_params_create(&pod);
+	VideoParams result = *static_cast<VideoParams *>(vp);
+	oakengine_video_params_free(vp);
+	return result;
+}
+
+VideoParams empty_video_params()
+{
+	oak_video_params pod = {};
+	return video_params_from_pod(pod);
+}
+
+Rational sequence_timebase(const void *sequence)
+{
+	oak_video_params vp = {};
+	if (oakengine_viewer_get_video_params(
+			reinterpret_cast<const OakEngineNode *>(sequence), 0, &vp) < 0 ||
+		vp.time_base_den <= 0) {
+		return Rational();
+	}
+	return Rational(vp.time_base_num, vp.time_base_den);
+}
+
+VideoParams viewer_output_video_params(const void *viewer, int index)
+{
+	oak_video_params vp;
+	if (oakengine_viewer_get_video_params(
+			reinterpret_cast<const OakEngineNode *>(viewer), index, &vp) < 0 ||
+		vp.width <= 0 || vp.height <= 0) {
+		return empty_video_params();
+	}
+	return video_params_from_pod(vp);
+}
+
+AudioParams viewer_output_audio_params(const void *viewer, int index)
+{
+	int sample_rate = 0;
+	int format = 0;
+	uint64_t channel_layout = 0;
+	if (oakengine_viewer_get_audio_params(
+			reinterpret_cast<const OakEngineNode *>(viewer), index, &sample_rate,
+			&channel_layout, &format) < 0 ||
+		sample_rate <= 0) {
+		return AudioParams();
+	}
+	return AudioParams(sample_rate, channel_layout,
+					   static_cast<SampleFormat::Format>(format));
+}
+
+} // namespace olive