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

src/render/c_api/cancelatom.cpp

@@ -133,3 +133,11 @@ int oakrender_cancelatom_heard_cancel(OakCancelAtom atom, int *heard)
 	*heard = c->heard_cancel() ? 1 : 0;
 	return OAKRENDER_OK;
 }
+
+olive::CancelAtom *oakrender_cancelatom_get_native(OakCancelAtom atom)
+{
+	if (!atom.ctx) {
+		return nullptr;
+	}
+	return &static_cast<CancelAtomBox *>(atom.ctx)->impl;
+}