refactor(node): de-Qt oaknode and wrap it in a pure C ABI

- copy engine/node (188 files) to src/node/src, de-Qt in waves:
  core infra (Node/Param/Value/Variant/mathtypes), project/serializer,
  block/output, color, effect leaves, generator, gizmo/plugins
- strip QObject/signals/slots: notifications move to the facade's
  oakengine_event channel, ownership becomes explicit (unique_ptr,
  add_keyframe/add_gizmo), sender() replaced by current_gizmo
- QVariant replaced by olive::Variant, Qt math types by POD mathtypes,
  QXmlStreamReader/Writer by oakcommon's expat-based classes
- sink VideoParams/SubtitleParams/LoopMode/ColorTransform to oakcommon
  (M3.5); polygon/text rasterization behind backend hooks
- pure C ABI in include/node + src/node/c_api (oaknode_ prefix,
  OAKNODE_E_* codes, undoable variants take OakUndoCommand out-params)
- fix Project::clear() root_ reset + disconnect assert, Sequence
  TrackList leak
- 96 gtest cases green in standalone build (build-oaknode)
- docs: signal/slot handling strategy + M3 implementation status

src/node/transition/pluginSupport/olivehost.h

@@ -0,0 +1,12 @@
+#pragma once
+// Transitional stub for engine/pluginSupport/olivehost.h (still Qt-based).
+// Only the surface oaknode uses. M9 replaces this with the real plugin
+// boundary.
+#include <string>
+#include "ofxhImageEffectAPI.h"
+#include "ofxhPluginCache.h"
+namespace olive {
+namespace plugin {
+inline void load_plugins(const std::string &) {}
+}
+}

src/node/transition/pluginSupport/oliveplugininstance.h

@@ -0,0 +1,30 @@
+#pragma once
+// Minimal syntax-check stub for engine/pluginSupport/oliveplugininstance.h
+// (real header is Qt-based, handled in a later milestone). Not in repo.
+#include <memory>
+
+#include "ofxhImageEffect.h"
+
+namespace olive
+{
+namespace plugin
+{
+class PluginNode;
+}
+
+class OlivePluginInstance : public OFX::Host::ImageEffect::Instance {
+public:
+	void setNode(std::shared_ptr<plugin::PluginNode> node)
+	{
+		node_ = node;
+	}
+	std::shared_ptr<plugin::PluginNode> node() const
+	{
+		return node_;
+	}
+
+private:
+	std::shared_ptr<plugin::PluginNode> node_;
+};
+
+}