Supply-chain internship project: find the stockout before a replenishment

Original invented practice inputs.

Task: An event-level stock ledger and an explanation of unmet demand.

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://www.sqlite.org/windowfunctions.html

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