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/dialog/export/codec/av1section.h

@@ -58,7 +58,7 @@ public:
 	AV1Section(QWidget *parent = nullptr);
 	AV1Section(int default_crf, QWidget *parent);
 
-	virtual void add_opts(EncodingParams *params) override;
+	virtual void add_opts(OakEngineEncodingParams *params) override;
 
 private:
 	QStackedWidget *compression_method_stack_;