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/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);