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/colorcodingapp.h

@@ -0,0 +1,75 @@
+/***
+
+  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/>.
+
+***/
+
+#ifndef OAK_COLORCODINGAPP_H
+#define OAK_COLORCODINGAPP_H
+
+#include <olive/core/core.h>
+#include <QString>
+#include <QVector>
+
+namespace olive
+{
+
+using namespace core;
+
+/**
+ * @brief App-side ColorCoding (moved from engine/ui/colorcoding.h)
+ *
+ * Provides the same static color-label mapping as the engine version but
+ * without QObject inheritance (no moc symbols). Only the static methods
+ * used by app code are included.
+ */
+class ColorCoding {
+public:
+	enum Code {
+		k_red,
+		k_maroon,
+		k_orange,
+		k_brown,
+		k_yellow,
+		k_olive,
+		k_lime,
+		k_green,
+		k_cyan,
+		k_teal,
+		k_blue,
+		k_navy,
+		k_pink,
+		k_purple,
+		k_silver,
+		k_gray
+	};
+
+	static QString get_color_name(int c);
+
+	static Color get_color(int c);
+
+	static Qt::GlobalColor get_ui_selector_color(const Color &c);
+
+	static const QVector<Color> &standard_colors();
+
+private:
+	static QVector<Color> colors;
+};
+
+} // namespace olive
+
+#endif // OAK_COLORCODINGAPP_H