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/task/error.h

@@ -0,0 +1,40 @@
+/***
+
+  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/>.
+
+***/
+
+#ifndef OAK_EDITOR_TASK_ERROR_H
+#define OAK_EDITOR_TASK_ERROR_H
+
+/**
+ * @brief Status and error codes shared by all oaktask C API families.
+ *
+ * Return-code convention (mirrors engine/include/oakengine/init.h):
+ * 0 (OAKTASK_OK) on success, a negative OAKTASK_E_* error code on
+ * failure. String getters return the required buffer size in bytes
+ * (including the terminating NUL) as a non-negative value instead.
+ */
+#define OAKTASK_OK 0 /**< Success. */
+#define OAKTASK_E_INVALID (-1) /**< NULL handle or invalid argument. */
+#define OAKTASK_E_STATE (-2) /**< Call not valid in the current state. */
+#define OAKTASK_E_FAILED (-3) /**< The underlying operation failed. */
+#define OAKTASK_E_NOT_FOUND (-4) /**< Index out of range / entry not found. */
+#define OAKTASK_E_NOMEM (-5) /**< Allocation failed. */
+#define OAKTASK_E_CANCELLED (-6) /**< The operation was cancelled. */
+
+#endif //OAK_EDITOR_TASK_ERROR_H