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/codec/decoder.h

@@ -25,6 +25,7 @@
 
 #include "error.h"
 #include "frame.h"
+#include "render/cancelatom.h"
 
 #ifdef __cplusplus
 extern "C" {
@@ -199,6 +200,38 @@ OAKCODEC_API int oakcodec_decoder_decode_audio(OakDecoder decoder, int in_num, i
 								  uint64_t channel_layout, float *buf,
 								  int buf_frames);
 
+/**
+ * @brief Conform the open stream's audio into per-channel pcm cache files
+ *        (Decoder::conform_audio()).
+ *
+ * `output_filenames` is an array of `filename_count` final per-channel
+ * paths. `sample_format` is olive::core::SampleFormat::Format as int.
+ * `cancelled` may be an empty OakCancelAtom (ctx == NULL).
+ *
+ * @return OAKCODEC_OK on success, OAKCODEC_E_STATE when no stream is
+ *         open, OAKCODEC_E_CANCELLED when cancelled, OAKCODEC_E_FAILED
+ *         otherwise.
+ */
+OAKCODEC_API int oakcodec_decoder_conform_audio(OakDecoder decoder,
+		const char *const *output_filenames, int filename_count,
+		int sample_rate, uint64_t channel_layout, int sample_format,
+		OakCancelAtom cancelled);
+
+/**
+ * @brief Image-sequence filename heuristics (Decoder::get_image_sequence_*).
+ *
+ * digit_count: number of trailing digits in the filename stem (0 = not an
+ * image sequence filename). index: the numeric value of those digits (-1
+ * when none). transform: substitute `number` into the digit field,
+ * two-stage string getter.
+ */
+OAKCODEC_API int oakcodec_decoder_get_image_sequence_digit_count(
+		const char *filename);
+OAKCODEC_API int64_t oakcodec_decoder_get_image_sequence_index(
+		const char *filename);
+OAKCODEC_API int oakcodec_decoder_transform_image_sequence_file_name(
+		const char *filename, int64_t number, char *buf, int buf_size);
+
 /**
  * @brief Human-readable detail of the last error on this decoder
  *        (buf/size string getter convention).