refactor(render,timeline,task): switch remaining handles to refcounted value structs

- oakrender: renderer/texture/frame/cache/colorprocessor/ticket/copier
  all become by-value {ctx, addref, release, abi_version} handles over a
  generic box; retain() folds into addref; new
  oakrender_cache_wrap_borrowed for native caches from oaknode
- oaktimeline: marker list/workarea borrowed handles become value
  handles (non-owning boxes) with explicit free
- oaktask: OakTaskTask becomes a value handle; ownership still moves
  to the manager on start (owns flag flips)
- consumers (timeline/task/codec sources) migrated; tests everywhere
  updated; suites green: render 45, timeline 117, task 106, node 96,
  common 193, codec 18, audio 36

src/task/c_api/manager.cpp

@@ -55,13 +55,13 @@ int oaktask_manager_count(void)
 	return olive::TaskManager::instance()->get_task_count();
 }
 
-OakTaskTask *oaktask_manager_at(int i)
+OakTaskTask oaktask_manager_at(int i)
 {
 	if (!olive::TaskManager::instance()) {
-		return NULL;
+		return OakTaskTask{};
 	}
 	olive::Task *task = olive::TaskManager::instance()->get_task_at(i);
-	// Borrowed wrapper: freeing it does not delete the task.
+	// Borrowed wrapper: releasing it does not delete the task.
 	return oaktask_capi::wrap_borrowed(task);
 }