Planning internship project: separate baseline demand from a promotion

Original invented practice inputs.

Task: A scenario forecast with an explicit promotion assumption and error-review loop.

Start with input.json and the CSV tables. Record your method before opening answer-key.json. Keep a changed input and explain how its result differs.

Technical reading:
https://support.microsoft.com/en-us/excel/sum-values-in-a-pivottable

A result from this fixture is independent practice work; describe only work you actually complete.
