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/timeruler/seekablewidget.h

@@ -27,6 +27,7 @@
 
 #include "widget/menu/menu.h"
 #include "widget/timebased/timebasedviewselectionmanager.h"
+#include "engineeventbridge.h"
 
 namespace olive
 {
@@ -169,6 +170,14 @@ private:
 
 	bool marker_editing_enabled_;
 
+	QVector<int64_t> marker_list_subs_;
+	bool marker_connects_done_ = false;
+
+	EngineEventBridge *bridge_ = nullptr;
+
+	int64_t workarea_range_sub_ = 0;
+	int64_t workarea_enabled_sub_ = 0;
+
 	QPolygon last_playhead_shape_;
 
 private slots: