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.

include/node/node.h

@@ -609,6 +609,39 @@ int oaknode_viewer_set_audio_params(OakNodeNode viewer,
  */
 int oaknode_node_find_input_footage(OakNodeNode node, OakNodeFootage *out);
 
+/**
+ * @brief Value of an input at a specific time (Node::get_value_at_time(),
+ * element -1). Same POD rules as oaknode_node_get_input().
+ */
+int oaknode_node_get_input_at_time(OakNodeNode node,
+								   const char *input_id, int64_t time_num,
+								   int64_t time_den, oaknode_value *out);
+
+/**
+ * @brief Set an input's value at a specific time with keyframe logic
+ *        (Node::set_value_at_time(), element -1, track 0,
+ *        insert_on_all_tracks_if_no_key = true). `*out_command` receives
+ *        an owned undo command handle.
+ */
+int oaknode_node_set_input_at_time_undoable(OakNodeNode node,
+		const char *input_id, int64_t time_num, int64_t time_den,
+		const oaknode_value *v, int track, OakUndoCommand *out_command);
+
+/**
+ * @brief Identity of the underlying node object as an opaque integer
+ *        (address-cast; for registry keys only, never dereference).
+ */
+uintptr_t oaknode_node_identity(OakNodeNode node);
+
+/**
+ * @brief Append a value-at-time set into an existing multi command
+ *        (same semantics as oaknode_node_set_input_at_time_undoable but
+ *        batches into `multi_command` from oakundo_command_init_multi()).
+ */
+int oaknode_node_set_input_at_time_into(OakNodeNode node,
+		const char *input_id, int64_t time_num, int64_t time_den,
+		const oaknode_value *v, int track, OakUndoCommand multi_command);
+
 /**
  * @brief Create a command that removes a node from its graph together
  * with its exclusive dependencies and disconnects its edges