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/timebased/timebasedview.h

@@ -23,6 +23,7 @@
 #define OAK_TIMELINEVIEWBASE_H
 
 #include <QGraphicsView>
+#include <cstdint>
 #include <vector>
 
 #include "core.h"
@@ -146,6 +147,8 @@ private:
 	double y_scale_;
 
 	ViewerOutput *viewer_;
+
+	int64_t viewer_sub_ = 0;
 };
 
 }