refactor(node): switch oaknode to refcounted value handles, migrate consumers

- all 15 OakNode* handle types become neutral by-value structs
  {ctx, addref, release, abi_version}; shared box in
  src/node/c_api/nodehandle.h with owns flag (borrowed accessors
  return non-owning boxes; graph insertion flips owns off)
- oaktimeline/oaktask/oakrender call sites and their own public
  headers migrated to value handles; identity comparisons in
  timeline/task now compare native pointers
- regressions green: oaknode 96, oaktimeline 117, oaktask 106,
  oakrender 44, oakcommon 193, oakcodec 18, oakaudio 36

include/task/project.h

@@ -41,18 +41,21 @@ extern "C" {
 /** @brief olive::ProjectLoadTask. */
 OakTaskTask *oaktask_create_project_load(const char *filename);
 
-/** @brief Take the loaded project (ownership transfer). */
-OakNodeProject *oaktask_load_take_project(OakTaskTask *t);
+/** @brief Take the loaded project (ownership transfer). Empty handle
+ *        (ctx == NULL) when the task has not succeeded or the project was
+ *        already taken. */
+OakNodeProject oaktask_load_take_project(OakTaskTask *t);
 
 /** @brief olive::ProjectSaveTask. `filename_or_NULL` overrides the
- *        project's own filename. */
-OakTaskTask *oaktask_create_project_save(OakNodeProject *project,
+ *        project's own filename. `project` is borrowed by the task. */
+OakTaskTask *oaktask_create_project_save(OakNodeProject project,
 										 const char *filename_or_NULL,
 										 int use_compression);
 
-/** @brief olive::ProjectImportTask. */
-OakTaskTask *oaktask_create_project_import(OakNodeFolder *folder,
-										   OakNodeProject *project,
+/** @brief olive::ProjectImportTask. `folder`/`project` are borrowed by
+ *        the task. */
+OakTaskTask *oaktask_create_project_import(OakNodeFolder folder,
+										   OakNodeProject project,
 										   const char *const *urls,
 										   int url_count);
 
@@ -61,8 +64,10 @@ OakUndoCommand oaktask_import_take_command(OakTaskTask *t);
 
 int oaktask_import_footage_count(OakTaskTask *t);
 
-/** @brief Borrowed footage handle at index, NULL when out of range. */
-OakNodeFootage *oaktask_import_footage_at(OakTaskTask *t, int index);
+/** @brief Footage handle at index (addref'd; release with
+ *        handle.release(handle.ctx) - box only, the project owns the
+ *        footage). Empty handle when out of range. */
+OakNodeFootage oaktask_import_footage_at(OakTaskTask *t, int index);
 
 int oaktask_import_invalid_count(OakTaskTask *t);
 
@@ -73,11 +78,13 @@ int oaktask_import_invalid_at(OakTaskTask *t, int index, char *buf,
 /** @brief olive::LoadOTIOTask. */
 OakTaskTask *oaktask_create_project_load_otio(const char *filename);
 
-/** @brief Take the loaded project (ownership transfer). */
-OakNodeProject *oaktask_load_otio_take_project(OakTaskTask *t);
+/** @brief Take the loaded project (ownership transfer). Empty handle
+ *        (ctx == NULL) when the task has not succeeded or the project was
+ *        already taken. */
+OakNodeProject oaktask_load_otio_take_project(OakTaskTask *t);
 
-/** @brief olive::SaveOTIOTask. */
-OakTaskTask *oaktask_create_project_save_otio(OakNodeProject *project,
+/** @brief olive::SaveOTIOTask. `project` is borrowed by the task. */
+OakTaskTask *oaktask_create_project_save_otio(OakNodeProject project,
 											  const char *filename);
 
 /**
@@ -89,13 +96,15 @@ typedef int (*oaktask_otio_import_confirm_fn)(
 void oaktask_load_otio_set_confirm_cb(oaktask_otio_import_confirm_fn fn,
 									  void *userdata);
 
-/** @brief olive::PreCacheTask. */
-OakTaskTask *oaktask_create_precache(OakNodeFootage *footage, int index,
-									 OakNodeSequence *sequence);
+/** @brief olive::PreCacheTask. `footage`/`sequence` are borrowed by the
+ *        task. */
+OakTaskTask *oaktask_create_precache(OakNodeFootage footage, int index,
+									 OakNodeSequence sequence);
 
-/** @brief olive::ExportTask (params POD from codec/encoder.h). */
-OakTaskTask *oaktask_create_export(OakNodeNode *viewer,
-								   OakNodeColorManager *color_manager,
+/** @brief olive::ExportTask (params POD from codec/encoder.h).
+ *        `viewer`/`color_manager` are borrowed by the task. */
+OakTaskTask *oaktask_create_export(OakNodeNode viewer,
+								   OakNodeColorManager color_manager,
 								   const oakcodec_encoding_params *params);
 
 /**