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/nodeviewscene.h

@@ -63,10 +63,14 @@ public:
 		return curved_edges_;
 	}
 
-public slots:
+public:
+	// Not slots: signatures use the engine C++ type Node*, which must not be
+	// exposed to MOC (it would pull Node::staticMetaObject across the ABI
+	// boundary). They are called directly, never used as connect() targets.
 	NodeViewContext *add_context(Node *node);
 	void remove_context(Node *node);
 
+public slots:
 	/**
    * @brief Set whether edges in this scene should be curved or not
    */