Rewrite Path Invariance page for layman readability

Plain English explanations, 6 AI model cards, scatter plot with
"Color by Topic"/"Color by AI Model" toggles, per-question agreement
bars, 4-step methodology explanation, and honest caveat about shared
training artifacts.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: Jamie Nucho