Chapter 7: Saturation at E₃

The enrichment ladder E₀ → E₁ → E₂ → E₃ was introduced in Part I’s opening chapters as the organisational spine of the entire series. Chapters 4–6 proved that the four layers are non-empty and strictly nested: each E_k contains genuinely new structure that E_k-1 does not. This chapter proves the other direction: a hypothetical layer E₄ collapses back into E₃. The enrichment ladder is complete at exactly four levels. This is the deepest structural claim in the series. It explains not merely that reality has four layers but why it cannot have a fifth.