Leetcode 2691. Immutability Helper

Implement an immutability-helper that, given a nested object/array and a spec of update commands (e.g., $set, $push, $unshift, $splice, $merge, $apply), returns a new data structure with the updates applied without mutating the input. The challenge is to recursively handle nested commands and various operations while copying only affected subtrees to preserve structural sharing and performance.