We identify, formalize, and provide three proven instances of a structural phenomenon we call the \emph{Shannon--Wakil effect}: an exponential configuration space undergoes forced dimensional reduction to a strict subspace, governed by a constant that is uniquely determined by the algebraic structure of the system and cannot be altered without changing that structure. The first two instances are both due to Shannon (1948). The Asymptotic Equipartition Property (AEP) shows that $|A|^n$ sequences concentrate onto a typical set of size $\approx 2^{nH}$, where entropy $H$ is forced by the source distribution. The Channel Coding Theorem shows that $2^{nR}$ codes can be reliably communicated only for rates $R < C$, where the channel capacity $C$ is forced by the channel. In each case an exponential space of configurations is forced by structure into a strict subspace governed by a uniquely determined constant. The third instance is the cascade moduli result of the companion paper [9]: primes modulo powers of 3 concentrate onto an effective subspace governed by level of distribution $\theta_W = 5/8$, forced by the algebraic structure of the Eisenstein integers $\mathbb{Z}[\omega]$---specifically the ramification $(3) = (1-\omega)^2$ and the cyclic group structure of $(\mathbb{Z}/3^K\mathbb{Z})^*$. We exhibit a confirmed non-instance: Sophie Germain prime pairs, whose coupled modular constraints confine the level of distribution to $\theta_{BV} = 1/2$, exactly as predicted by the $k$-hierarchy $\theta_k = (2^k - k)/2^k$. The framework is therefore falsifiable: it correctly identifies which prime constellations do and do not exhibit the effect. We formalize the pattern through five structural correspondences, state the Constitutional Forcing Conjecture with categorical language, and identify an open prediction at $k=4$.