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/task/c_api/manager.cpp

@@ -0,0 +1,73 @@
+/***
+
+  Oak Video Editor - Non-Linear Video Editor
+  Copyright (C) 2026 Oak Team
+
+  This program is free software: you can redistribute it and/or modify
+  it under the terms of the GNU General Public License as published by
+  the Free Software Foundation, either version 3 of the License, or
+  (at your option) any later version.
+
+  This program is distributed in the hope that it will be useful,
+  but WITHOUT ANY WARRANTY; without even the implied warranty of
+  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+  GNU General Public License for more details.
+
+  You should have received a copy of the GNU General Public License
+  along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+***/
+
+#include "task/manager.h"
+
+#include "../src/codecbridge.h"
+#include "../src/taskmanager.h"
+#include "taskhandle.h"
+
+int oaktask_manager_init(void)
+{
+	if (olive::TaskManager::instance()) {
+		return OAKTASK_E_STATE;
+	}
+
+	olive::TaskManager::create_instance();
+	olive::register_codec_task_submitter();
+	return OAKTASK_OK;
+}
+
+void oaktask_manager_shutdown(void)
+{
+	olive::unregister_codec_task_submitter();
+	olive::TaskManager::destroy_instance();
+}
+
+int oaktask_register_codec_submitter(void)
+{
+	olive::register_codec_task_submitter();
+	return OAKTASK_OK;
+}
+
+int oaktask_manager_count(void)
+{
+	if (!olive::TaskManager::instance()) {
+		return OAKTASK_E_STATE;
+	}
+	return olive::TaskManager::instance()->get_task_count();
+}
+
+OakTaskTask *oaktask_manager_at(int i)
+{
+	if (!olive::TaskManager::instance()) {
+		return NULL;
+	}
+	olive::Task *task = olive::TaskManager::instance()->get_task_at(i);
+	// Borrowed wrapper: freeing it does not delete the task.
+	return oaktask_capi::wrap_borrowed(task);
+}
+
+void oaktask_manager_delete_finished(void)
+{
+	if (olive::TaskManager::instance()) {
+		olive::TaskManager::instance()->delete_finished();
+	}
+}