Innmind/BlackBox

AI Summary: The task is to add the `NoDiscard` attribute to methods within the BlackBox PHP library that do not modify the object's state (mutation-free). This is based on the PHP RFC proposing the `NoDiscard` attribute to mark return values as important, preventing accidental discarding of results from functions that are intended to produce a meaningful output. The goal is to improve code clarity and prevent potential bugs by explicitly indicating the intent of these methods.