fix: classify χ = ½ as critical balance

Adversarial verification found χ = ½ was unclassified.
Added: χ = ½ → critical balance (compressive = anti-compressive)

docs/PURE_FORMULAS.md

@@ -64,6 +64,8 @@ $$\chi > \frac{1}{2} \implies \text{compressive}$$
 
 $$\chi < \frac{1}{2} \implies \text{anti-compressive}$$
 
+$$\chi = \frac{1}{2} \implies \text{critical balance}$$
+
 ---
 
 ## 6. Q16_16