refactor(undo): switch oakundo to refcounted value handles, migrate consumers

- OakUndoCommand/OakUndoStack become neutral by-value handles
  {ctx, addref, release, abi_version}; a handle is a shared_ptr
  equivalent at the ABI level, internals untouched (box owns/observes
  flag; containers adopt, boxes created by factories own)
- oaknode command factories and undoable variants return/write value
  handles; timeline command classes hold value handles; task import
  take_command returns a value handle
- tests updated everywhere; suites green: oakundo 22, oaktimeline 117,
  oaktask 106, oaknode 96, oakrender 44, oakcodec 18, oakcommon 193,
  oakaudio 36

include/timeline/edit.h

@@ -41,60 +41,60 @@ extern "C" {
  */
 
 /** @brief olive::TimelineAddTrackCommand. */
-OakUndoCommand *oaktimeline_add_track_command(OakNodeTrackList *list);
+OakUndoCommand oaktimeline_add_track_command(OakNodeTrackList *list);
 
 /** @brief olive::TimelineRemoveTrackCommand. */
-OakUndoCommand *oaktimeline_remove_track_command(OakNodeTrack *track);
+OakUndoCommand oaktimeline_remove_track_command(OakNodeTrack *track);
 
 /** @brief olive::TrackPlaceBlockCommand. */
-OakUndoCommand *oaktimeline_place_block_command(OakNodeTrackList *list,
+OakUndoCommand oaktimeline_place_block_command(OakNodeTrackList *list,
 												int track_index,
 												OakNodeBlock *block,
 												int64_t in_num,
 												int64_t in_den);
 
 /** @brief olive::TrackReplaceBlockWithGapCommand. */
-OakUndoCommand *oaktimeline_replace_block_with_gap_command(
+OakUndoCommand oaktimeline_replace_block_with_gap_command(
 	OakNodeTrack *track, OakNodeBlock *block);
 
 /**
  * @brief olive::BlockTrimCommand. `mode` is an OakTimelineMovementMode
  * value (k_trim_in / k_trim_out).
  */
-OakUndoCommand *oaktimeline_trim_command(OakNodeTrack *track,
+OakUndoCommand oaktimeline_trim_command(OakNodeTrack *track,
 										 OakNodeBlock *block,
 										 int64_t new_length_num,
 										 int64_t new_length_den, int mode);
 
 /** @brief olive::BlockSplitCommand on a set of blocks at one point. */
-OakUndoCommand *oaktimeline_split_command(OakNodeBlock *const *blocks,
+OakUndoCommand oaktimeline_split_command(OakNodeBlock *const *blocks,
 										  int count, int64_t point_num,
 										  int64_t point_den);
 
 /** @brief olive::BlockSplitPreservingLinksCommand. */
-OakUndoCommand *oaktimeline_split_preserving_links_command(
+OakUndoCommand oaktimeline_split_preserving_links_command(
 	OakNodeBlock *const *blocks, int count, const int64_t *point_nums,
 	const int64_t *point_dens, int time_count);
 
 /** @brief olive::TimelineRippleDeleteGapsAtRegionsCommand. */
-OakUndoCommand *oaktimeline_ripple_delete_gaps_command(
+OakUndoCommand oaktimeline_ripple_delete_gaps_command(
 	OakNodeSequence *sequence, const int64_t *in_nums,
 	const int64_t *in_dens, const int64_t *out_nums,
 	const int64_t *out_dens, OakNodeTrack *const *tracks, int range_count);
 
 /** @brief olive::TrackSlideCommand. */
-OakUndoCommand *oaktimeline_slide_command(
+OakUndoCommand oaktimeline_slide_command(
 	OakNodeTrack *track, OakNodeBlock *const *blocks, int block_count,
 	OakNodeBlock *in_adjacent, OakNodeBlock *out_adjacent,
 	int64_t movement_num, int64_t movement_den);
 
 /** @brief olive::TrackRippleRemoveAreaCommand. */
-OakUndoCommand *oaktimeline_ripple_remove_area_command(
+OakUndoCommand oaktimeline_ripple_remove_area_command(
 	OakNodeTrack *track, int64_t in_num, int64_t in_den, int64_t out_num,
 	int64_t out_den);
 
 /** @brief olive::TrackListInsertGaps. */
-OakUndoCommand *oaktimeline_insert_gaps_command(OakNodeTrackList *list,
+OakUndoCommand oaktimeline_insert_gaps_command(OakNodeTrackList *list,
 												int64_t point_num,
 												int64_t point_den,
 												int64_t length_num,