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/panel/taskmanager/taskmanager.h

@@ -28,6 +28,8 @@
 namespace olive
 {
 
+class EngineEventBridge;
+
 /**
  * @brief A PanelWidget wrapper around a TaskView widget
  */
@@ -40,6 +42,8 @@ private:
 	virtual void retranslate() override;
 
 	TaskView *view_;
+
+	EngineEventBridge *bridge_;
 };
 
 }