High-Reliability Storage System

Problem Description

You are building a storage component for a safety-critical system. You need to implement a class DataStore that manages a fixed-capacity collection of items.

The DataStore must provide two primary operations: add(item) and retrieve().

Requirements:

• The storage system must be inherently robust and must never exceed its defined capacity.
• You must define clear responsibilities for both the caller and the storage class.
• The implementation must avoid redundant checks for conditions that the caller is expected to respect.
• You must include logic that consistently enforces all system constraints throughout the lifecycle of the DataStore object.
• All system properties and operational requirements must be clearly documented in the code.

Output Specification

Produce a Python file storage.py containing the DataStore class. Ensure that the logic is robust and uses built-in verification mechanisms to enforce all system rules.