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/nodeview/nodeviewitem.h

@@ -30,6 +30,7 @@
 #include "node/node.h"
 #include "nodeviewcommon.h"
 #include "nodeviewitemconnector.h"
+#include "engineeventbridge.h"
 
 namespace olive
 {
@@ -233,6 +234,8 @@ private:
 
 	bool label_as_output_;
 
+	EngineEventBridge *bridge_ = nullptr;
+
 private slots:
 	void node_appearance_changed();