Design an API and use standard library features where appropriate.
Implement a small in-memory key-value API supporting set, get, delete, and list operations. Keys are strings and values are JSON-compatible primitives. set must update an existing key without changing its original listing position.
Input/output contract: process_operations(operations) receives a list of operation objects and returns one result per operation. set returns "ok", get returns the stored value or None, delete returns a Boolean, and list returns keys in insertion order.
def process_operations(operations):