build: split the engine into liboakengine.so; worker drops the UI entirely

Physical split: app/{audio,cli,codec,common,config,node,pluginSupport,
render,task,timeline,undo,tool,shaders} plus coreengine, version and
ui/icons+colorcoding move to a new top-level engine/ tree, built as
liboakengine.so (shared). The render backends (oakgl/oakvulkan) move
with it and link the engine library instead of embedding a static
render-core subset (libolive-rendercore is gone).

- oak-render-worker now links liboakengine instead of the whole
  libolive-editor object set: 336MB -> 2.9MB, no Qt Widgets UI
- the editor links liboakengine for the engine and keeps only UI
  objects in libolive-editor
- install/packaging: GNUInstallDirs libdir on Linux, bundle copy on
  macOS, oakengine.dll staged for NSIS, AppImage validation entry
- fix backend lookup for the new layout: DynamicRenderer searched
  ../app but backends now live in engine/; a stale pre-split liboakgl
  in the build tree got dlopened instead, re-initialized and later
  destroyed the interposed engine statics (full-suite segfault at
  DialogSequenceParameterTab, found via gdb watchpoint)

engine/pluginSupport/olivehost.h

@@ -0,0 +1,105 @@
+/*
+ * Oak Video Editor - Non-Linear Video Editor
+ * Copyright (C) 2025 Olive CE Team
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+#ifndef OAK_OLIVE_HOST_H
+#define OAK_OLIVE_HOST_H
+#include "node/plugins/plugin.h"
+#include "ofxhHost.h"
+#include "ofxhImageEffectAPI.h"
+#include "ofxCore.h"
+#include "ofxhImageEffect.h"
+
+#include <QVariant>
+#include <cstdint>
+#include <QString>
+#include <QMap>
+#include <any>
+#include <list>
+#include <memory>
+#include <qlist.h>
+namespace olive
+{
+namespace plugin
+{
+enum class HostMessageType { error, warning, message };
+struct HostPersistentMessage {
+	HostMessageType type;
+	QString message;
+};
+
+void load_plugins(QString path);
+class OliveHost : public OFX::Host::ImageEffect::Host {
+public:
+	OliveHost();
+	~OliveHost() override;
+	void destroy_instance(OFX::Host::ImageEffect::Instance *instance);
+
+	bool pluginSupported(OFX::Host::ImageEffect::ImageEffectPlugin *plugin,
+						 std::string &reason) const override
+	{
+		if (!plugin) {
+			reason = "null plugin";
+			return false;
+		}
+		if (plugin->getContexts().empty()) {
+			reason = "no supported contexts (describe failed)";
+			return false;
+		}
+		return true;
+	};
+
+	OFX::Host::ImageEffect::Instance *
+	newInstance(void *client_data,
+				OFX::Host::ImageEffect::ImageEffectPlugin *plugin,
+				OFX::Host::ImageEffect::Descriptor &desc,
+				const std::string &context) override;
+
+	std::shared_ptr<OFX::Host::ImageEffect::Descriptor>
+	makeDescriptor(OFX::Host::ImageEffect::ImageEffectPlugin *plugin) override;
+
+	std::shared_ptr<OFX::Host::ImageEffect::Descriptor>
+	makeDescriptor(const OFX::Host::ImageEffect::Descriptor &root_context,
+				   OFX::Host::ImageEffect::ImageEffectPlugin *plugin) override;
+
+	std::shared_ptr<OFX::Host::ImageEffect::Descriptor>
+	makeDescriptor(const std::string &bundle_path,
+				   OFX::Host::ImageEffect::ImageEffectPlugin *plugin) override;
+	/// vmessage
+	virtual OfxStatus vmessage(const char *type, const char *id,
+							   const char *format, va_list args);
+
+	/// vmessage
+	virtual OfxStatus setPersistentMessage(const char *type, const char *id,
+										   const char *format, va_list args);
+	/// vmessage
+	virtual OfxStatus clearPersistentMessage();
+
+#ifdef OFX_SUPPORTS_OPENGLRENDER
+	/// @see OfxImageEffectOpenGLRenderSuiteV1.flushResources()
+	virtual OfxStatus flushOpenGLResources() const
+	{
+		return kOfxStatFailed;
+	};
+#endif
+private:
+	QList<std::shared_ptr<OFX::Host::ImageEffect::Descriptor>> descriptors_;
+	QList<std::shared_ptr<OFX::Host::ImageEffect::Instance>> instances_;
+	QList<HostPersistentMessage> persistent_messages_;
+};
+}
+}
+#endif