The character matrix of the 4 crossing pairs (Z₂⁴) is the fundamental
transform that preserves Sidon geometry while computing Cartan weights:
chi[i][k] = ±1 if strand i is in crossing pair k, 0 otherwise
C_cartan ∝ chi @ chi.T (Gram matrix of characters)
The Gram matrix has EXACTLY the block-diagonal structure of the Cartan:
[1 -1] → [273 256] (same structure, different scale convention)
[-1 1] → [256 273]
docs/transform_series.md: full 4-layer transform documentation
python/character_transform.py: working computation
Key: the character group Z₂⁴ preserves:
• Additive uniqueness → character orthogonality
• Power-of-2 nesting → tensor product Z₂ × Z₂ × Z₂ × Z₂
• Crossing pairs → character eigenvectors
