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/common/xmlutilsapp.cpp

@@ -0,0 +1,47 @@
+/***
+
+  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/>.
+
+***/
+
+// App-side implementation of xml_read_next_start_element
+// Provides a local definition so the app doesn't import this from liboakengine.
+
+#include "common/xmlutils.h"
+
+namespace olive
+{
+
+bool xml_read_next_start_element(QXmlStreamReader *reader,
+								CancelAtom *cancel_atom)
+{
+	QXmlStreamReader::TokenType token;
+
+	while ((token = reader->readNext()) != QXmlStreamReader::Invalid &&
+		   token != QXmlStreamReader::EndDocument &&
+		   (!cancel_atom || !cancel_atom->is_cancelled())) {
+		if (reader->isEndElement()) {
+			return false;
+		} else if (reader->isStartElement()) {
+			return true;
+		}
+	}
+
+	return false;
+}
+
+}