refactor(plugin): de-Qt oakplugin and wrap it in a pure C ABI

- de-Qt src/plugin/src (olivehost/oliveplugininstance/oliveclip/
  paraminstance/image/pluginprogressreporter); QMessageBox/
  QApplication replaced by facade callbacks
- new C ABI in include/plugin/{error,host,instance}.h with
  refcounted OakPluginInstance value handle
- paraminstance bridges via oaknode C ABI (OakNodeNode value handle,
  oaknode_node_identity registry); undo via oakundo C ABI
- oliveclip textures are OakRenderTexture value handles; oakrender
  gains texture/copier/ticket C API additions
- oaknode gains get_input_at_time/set_input_at_time_undoable/
  node_identity/sequence_from_node/sequence_set_default_parameters/
  find_input_footage
- move avframeptr.h to src/common/src (shared by codec and render)
- node transition pluginSupport stubs bridge the real oakplugin
  headers; all oaknode-building standalone trees add src/plugin and
  link oakplugin into their test binaries
- plugin tests self-sufficient (ipc shim, OfxHost force_load,
  PRE_TEST discovery, OCIO env); timeline standalone gains
  render/c_api
- restore ffmpegdecoder.cpp in src/codec/src/ffmpeg/CMakeLists.txt
  (dropped in 3d004c081, caused jump-to-0 in decoder/encoder tests)

All six standalone trees green: codec 22, audio 40, task 110,
render 49, timeline 121, plugin 100.

src/node/transition/pluginSupport/olivehost.h

@@ -1,12 +1,3 @@
 #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 &) {}
-}
-}
+// M9 收尾：桥接到 oakplugin 真身
+#include "../../../plugin/src/olivehost.h"

src/node/transition/pluginSupport/oliveplugininstance.h

@@ -1,30 +1,3 @@
 #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_;
-};
-
-}
+// M9 收尾：桥接到 oakplugin 真身
+#include "../../../plugin/src/oliveplugininstance.h"