Artifact-cause tracer (guided pipeline-stage diagnostic)

A small, dependency-free Python expert-system that runs the article's diagnostic flow. It encodes a knowledge base of the common video artifacts — each with its pause-test class (spatial/temporal), what it is attached to (block grid / content / screen), its most likely pipeline stage and root cause, the fix at the cause, and which metric would (or would not) have caught it. Two modes: (1) INTERACTIVE — a guided questionnaire (pause test? on a block grid? follows content/edges? only in motion? ap