refactor(task): de-Qt oaktask base + project tasks, wire codec task submitter

- Task base: Qt signals become lifecycle listeners (the async-command
  callback exception), cancellation uses oakrender's OakCancelAtom C
  handle; TaskManager runs std::thread workers, no signals
- ConformTask/ProxyTask implement oakcodec's submit-callback contract
  (synchronous interim); register_codec_task_submitter() closes the
  conform/proxy loop - conform of demo.mp4 produces pcm caches in tests
- ProjectImportTask/ProjectLoadTask/ProjectSaveTask over the oaknode
  C ABI; image-sequence confirmation becomes a facade callback
  (default: not a sequence)
- C API additions needed by oaktask: oaknode serializer file-level
  save/load (auto-initializing), footage video-params/cancel-atom/
  as-node, folder add-child command factory; oakcodec decoder
  conform_audio + image-sequence helpers; oakrender cancelatom
  get_native
- fix double-ownership: submitting a task to the manager transfers
  ownership, freeing its handle only releases the wrapper
- C ABI in include/task (task/manager/project, OakTaskTask opaque
  handle + lifecycle subscribe), every function tested (103 cases in
  build-oaktask incl. regressions)
- RenderTask family and OTIO tasks follow in separate commits

include/node/folder.h

@@ -22,6 +22,7 @@
 #define OAK_EDITOR_NODE_FOLDER_H
 
 #include "node/error.h"
+#include "undo/undocommand.h"
 #include "node/project.h"
 
 #ifdef __cplusplus
@@ -73,6 +74,19 @@ OakNodeNode *oaknode_folder_child_at(const OakNodeFolder *folder, int index);
  */
 int oaknode_folder_add_child(OakNodeFolder *folder, OakNodeNode *child);
 
+/**
+ * @brief Borrowed cast from a folder handle to its node handle.
+ * NULL for NULL.
+ */
+OakNodeNode *oaknode_folder_as_node(OakNodeFolder *folder);
+
+/**
+ * @brief Create an undoable FolderAddChild command. Owned; free with
+ *        oakundo_command_free().
+ */
+OakUndoCommand *oaknode_command_create_folder_add_child(
+	OakNodeFolder *folder, OakNodeNode *child);
+
 /**
  * @brief Remove `child` from `folder` without deleting it (live,
  * non-undoable; executes Folder::RemoveElementCommand::redo()).

include/node/footage.h

@@ -23,7 +23,12 @@
 
 #include <stdint.h>
 
+#include "common/videoparams.h"
 #include "node/error.h"
+// NOTE: quoted-relative to bypass the "render/cancelatom.h" transition
+// bridge (oakrender's C++ olive::CancelAtom) that shadows the C ABI
+// header on oaknode's include path.
+#include "../../include/render/cancelatom.h"
 #include "node/project.h"
 
 #ifdef __cplusplus
@@ -60,6 +65,12 @@ typedef struct OakNodeFootage OakNodeFootage;
 OakNodeFootage *oaknode_footage_create(OakNodeProject *project,
 									   const char *filename);
 
+/**
+ * @brief Borrowed cast from a footage handle to its node handle.
+ * NULL for NULL.
+ */
+OakNodeNode *oaknode_footage_as_node(OakNodeFootage *footage);
+
 /**
  * @brief Current media path (Footage::filename()). Two-stage string getter.
  *
@@ -192,6 +203,36 @@ int oaknode_footage_set_proxy(OakNodeFootage *footage, const char *path,
  */
 int oaknode_footage_clear_proxy(OakNodeFootage *footage);
 
+/**
+ * @brief Video stream parameters as an oakcommon video-params handle
+ * (ViewerOutput::get_video_params()). `out` receives a handle with
+ * reference count 1 (release with oakcommon_videoparams_free()).
+ * OAKNODE_E_NOT_FOUND for an out-of-range index.
+ */
+int oaknode_footage_get_video_params(OakNodeFootage *footage, int index,
+									 OakVideoParams *out);
+
+/**
+ * @brief Set a video stream's parameters from an oakcommon handle
+ * (ViewerOutput::set_video_params()).
+ */
+int oaknode_footage_set_video_params(OakNodeFootage *footage, int index,
+									 const OakVideoParams *params);
+
+/**
+ * @brief Video length as a rational pair (ViewerOutput::get_video_length()).
+ */
+int oaknode_footage_get_video_length(OakNodeFootage *footage,
+									 int64_t *out_num, int64_t *out_den);
+
+/**
+ * @brief Set the footage's cancellation atom used during probing
+ * (Footage::set_cancel_pointer()). `atom` may be an empty OakCancelAtom
+ * (ctx == NULL) to clear.
+ */
+int oaknode_footage_set_cancel_atom(OakNodeFootage *footage,
+									OakCancelAtom atom);
+
 #ifdef __cplusplus
 }
 #endif

include/node/serializer.h

@@ -242,3 +242,44 @@ int oaknode_serializer_loaddata_connection_at(
 #endif
 
 #endif //OAK_EDITOR_NODE_SERIALIZER_H
+
+/**
+ * @brief Result codes for file-level save/load (mirror
+ *        ProjectSerializer::ResultCode; pinned by test).
+ */
+enum OakNodeSerializerResultCode {
+	OAKNODE_SERIALIZER_RESULT_SUCCESS = 0,
+	OAKNODE_SERIALIZER_RESULT_PROJECT_TOO_OLD = 1,
+	OAKNODE_SERIALIZER_RESULT_PROJECT_TOO_NEW = 2,
+	OAKNODE_SERIALIZER_RESULT_UNKNOWN_VERSION = 3,
+	OAKNODE_SERIALIZER_RESULT_FILE_ERROR = 4,
+	OAKNODE_SERIALIZER_RESULT_XML_ERROR = 5,
+	OAKNODE_SERIALIZER_RESULT_OVERWRITE_ERROR = 6,
+	OAKNODE_SERIALIZER_RESULT_NO_DATA = 7
+};
+
+/**
+ * @brief Save a project to a file (ProjectSerializer::save(), project
+ *        type, optional OVEC compression). Layout data is not serialized
+ *        through this API (app-layer concern, see oakstorage/M10).
+ *
+ * @param out_code Receives an OakNodeSerializerResultCode (may be NULL).
+ * @param details Optional two-stage buffer for the result details
+ *        string (e.g. the fallback filename on overwrite errors).
+ * @return OAKNODE_OK when the result code is
+ *         OAKNODE_SERIALIZER_RESULT_SUCCESS, OAKNODE_E_FAILED otherwise
+ *         (details in out_code/details), OAKNODE_E_INVALID for NULL args.
+ */
+int oaknode_serializer_save_to_file(OakNodeProject *project,
+		const char *filename, int use_compression, int *out_code,
+		char *details, int details_size);
+
+/**
+ * @brief Load a project from a file into `project`
+ *        (ProjectSerializer::load(), project type).
+ *
+ * Same return/out-param convention as oaknode_serializer_save_to_file().
+ */
+int oaknode_serializer_load_from_file(OakNodeProject *project,
+		const char *filename, int *out_code, char *details,
+		int details_size);