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/curvewidget/curvewidget.h

@@ -62,7 +62,10 @@ public:
 
 	virtual bool paste() override;
 
-public slots:
+public:
+	// Not a slot: signature uses the engine C++ type Node*, which must not be
+	// exposed to MOC (it would pull Node::staticMetaObject across the ABI
+	// boundary). All connections use new-style member-function syntax.
 	void set_nodes(const QVector<Node *> &nodes);
 
 protected: