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/node/keyframe.h

@@ -109,7 +109,7 @@ int oaknode_keyframe_set_time(OakNodeKeyframe *keyframe, int64_t time_num,
  */
 int oaknode_keyframe_set_time_undoable(OakNodeKeyframe *keyframe,
 									   int64_t time_num, int64_t time_den,
-									   OakUndoCommand **out_command);
+									   OakUndoCommand *out_command);
 
 /**
  * @brief Read the keyframe's value mapped into `out`. Values without a
@@ -132,7 +132,7 @@ int oaknode_keyframe_set_value(OakNodeKeyframe *keyframe,
  */
 int oaknode_keyframe_set_value_undoable(OakNodeKeyframe *keyframe,
 										const oaknode_value *v,
-										OakUndoCommand **out_command);
+										OakUndoCommand *out_command);
 
 /**
  * @brief Read a string value. Two-stage getter.
@@ -154,7 +154,7 @@ int oaknode_keyframe_set_value_string(OakNodeKeyframe *keyframe,
  */
 int oaknode_keyframe_set_value_string_undoable(OakNodeKeyframe *keyframe,
 											   const char *value,
-											   OakUndoCommand **out_command);
+											   OakUndoCommand *out_command);
 
 /**
  * @brief The keyframe's interpolation type (oaknode_keyframe_type).
@@ -171,7 +171,7 @@ int oaknode_keyframe_set_type(OakNodeKeyframe *keyframe, int type);
  * @brief Create a set-type command (same semantics as the live variant).
  */
 int oaknode_keyframe_set_type_undoable(OakNodeKeyframe *keyframe, int type,
-									   OakUndoCommand **out_command);
+									   OakUndoCommand *out_command);
 
 /**
  * @brief A bezier control point (`handle` is an
@@ -192,7 +192,7 @@ int oaknode_keyframe_set_bezier_control(OakNodeKeyframe *keyframe, int handle,
  */
 int oaknode_keyframe_set_bezier_control_undoable(OakNodeKeyframe *keyframe,
 												 int handle, double x, double y,
-												 OakUndoCommand **out_command);
+												 OakUndoCommand *out_command);
 
 /**
  * @brief The keyframe's track index.