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/diskcache/diskcachedialog.cpp

@@ -26,6 +26,8 @@
 #include <QLabel>
 #include <QMessageBox>
 
+#include "oakengine/disk.h"
+
 namespace olive
 {
 
@@ -109,7 +111,7 @@ void DiskCacheDialog::clear_disk_cache(const QString &path, QWidget *parent,
 		if (clear_btn)
 			clear_btn->setEnabled(false);
 
-		if (DiskManager::instance()->clear_disk_cache(path)) {
+		if (oakengine_disk_clear_cache(path.toUtf8().constData())) {
 			if (clear_btn)
 				clear_btn->setText(tr("Disk Cache Cleared"));
 		} else {