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/task.h

@@ -0,0 +1,97 @@
+/***
+
+  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_TASK_H
+#define OAK_EDITOR_TASK_TASK_H
+
+#include <stdint.h>
+
+#include "task/error.h"
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+/**
+ * @brief Opaque handle to a background task (olive::Task).
+ *
+ * Tasks are created through the factories in task/project.h (and future
+ * family headers) and must be released with oaktask_task_free().
+ */
+typedef struct OakTaskTask OakTaskTask;
+
+/** @brief Lifecycle event ids for oaktask_task_subscribe(). */
+enum OakTaskEvent {
+	OAKTASK_EVENT_STARTED = 0,
+	OAKTASK_EVENT_PROGRESS = 1,
+	OAKTASK_EVENT_FINISHED = 2
+};
+
+/**
+ * @brief Event callback (async command return channel, 01 §4 exception).
+ *
+ * For OAKTASK_EVENT_FINISHED, `value` is 1.0 on success / 0.0 on failure;
+ * for OAKTASK_EVENT_PROGRESS it is 0..1; for OAKTASK_EVENT_STARTED it is
+ * the start time in milliseconds.
+ */
+typedef void (*oaktask_event_fn)(int event_id, double value,
+								 void *userdata);
+
+/**
+ * @brief Free a task. No-op on NULL. The task must not be running on the
+ *        manager (oaktask_task_cancel + wait first if it is).
+ */
+void oaktask_task_free(OakTaskTask *t);
+
+/** @brief Run synchronously in the calling thread. 1 = succeeded. */
+int oaktask_task_start_sync(OakTaskTask *t);
+
+/** @brief Run asynchronously on the task manager. */
+int oaktask_task_start(OakTaskTask *t);
+
+int oaktask_task_cancel(OakTaskTask *t);
+
+/** @brief Wait for an asynchronously started task. */
+int oaktask_task_wait(OakTaskTask *t);
+
+int oaktask_task_is_finished(const OakTaskTask *t);
+
+int oaktask_task_succeeded(const OakTaskTask *t);
+
+/** @brief Two-stage string getters. */
+int oaktask_task_title(OakTaskTask *t, char *buf, int buf_size);
+int oaktask_task_error(OakTaskTask *t, char *buf, int buf_size);
+
+/**
+ * @brief Subscribe to lifecycle events (returns a subscription id >= 0,
+ *        or a negative error code). One-shot per event stream: the
+ *        subscription is dropped after OAKTASK_EVENT_FINISHED.
+ */
+int64_t oaktask_task_subscribe(OakTaskTask *t, oaktask_event_fn fn,
+							   void *userdata);
+
+/** @brief Alive-count for leak assertions in tests. */
+int oaktask_debug_alive_count(void);
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif //OAK_EDITOR_TASK_TASK_H