Feature: Counter-example illustrations and JEE-related utilities

[yueswater]

Overview

Add a set of utilities and illustrations to support counter-example teaching
and topics related to the Journal of Economics Education (JEE).

Proposed additions

• Counter-example illustration helper — a reusable function for plotting
  counter-examples (e.g. violations of standard assumptions).
• Price decomposition — visualize the decomposition of price effects.
• Functional form illustrations
  • Log-additive utility / production forms
  • Log-power forms
• CES transition with $\rho$ — animate or illustrate how CES behavior
  shifts as the substitution parameter $\rho$ varies across limiting cases
  (Leontief, Cobb–Douglas, linear).