Extend the public rubico/x/defaultsDeep helper so callers can choose what counts as an already-present…

Extend the public rubico/x/defaultsDeep helper so callers can choose what counts as an already-present…: a task in MiMo-V2.6-RL-harbor-code: MiMo-V2.6-RL Code (Harbor) (Harbor dataset). Support defaultsDeep(defaultCollection, isPresent)(value) , where isPresent is a synchronous predicate receiving…