Legacy debugging set: seven catalog helpers, one bug each

Context

This set follows the debugging section candidates reported in Amazon's online assessment from about 2019 to 2023: seven short functions, each meant to do what its docstring says, each with exactly one bug. That section used C, C++ or Java; this set is in Python. Recent Amazon assessments use a different format, so treat this as practice for the skill: finding a fault fast against a written spec.

Each item is graded on its own, so every one you fix counts. The bug is in the function's code, never in its docstring; the fix is usually a line or two.

Items

Item	File	Function
Q1	catalog/skus.py	normalize_sku
Q2	catalog/ratings.py	rating_summary
Q3	catalog/pages.py	page
Q4	catalog/prices.py	in_price_range
Q5	catalog/sellers.py	top_sellers
Q6	catalog/inventory.py	merge_inventory
Q7	catalog/duplicates.py	has_duplicates

Notes

• Run pytest tests/visible. Seven tests fail on the current code, one per item.
• Practice set modelled on candidate reports of the debugging section in Amazon's earlier online assessment. Original StackYard code, not the actual questions; not affiliated with Amazon.