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/render/cancelatom.h

@@ -29,6 +29,14 @@
 extern "C" {
 #endif
 
+#ifdef __cplusplus
+} /* extern "C" */
+
+namespace olive { class CancelAtom; }
+
+extern "C" {
+#endif
+
 /**
  * @brief Current ABI version stamped into every oakrender handle.
  *
@@ -108,8 +116,14 @@ int oakrender_cancelatom_is_cancelled(OakCancelAtom atom, int *cancelled);
  *         NULL out parameter.
  */
 int oakrender_cancelatom_heard_cancel(OakCancelAtom atom, int *heard);
-
 #ifdef __cplusplus
+
+/**
+ * @brief Borrowed access to the underlying C++ atom (C++ only, for
+ *        adapter layers). Valid while the handle is held. NULL-safe.
+ */
+olive::CancelAtom *oakrender_cancelatom_get_native(OakCancelAtom atom);
+
 }
 #endif