DmcsLimit

The Gaussian limit: exact tie to cv_rate, and convergence from below

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Test	What it does	Result
test_holevo_alphabet	Four states cap $I(X;Y)$ at 2 bits where the unbounded form claims 3.459 at the same SNR, lowering the key without touching Eve's bound.	✅ pass
test_holevo_matches	Fed $\sqrt{T(V^2-1)}$ and an unbounded alphabet, dm_holevo reproduces cv_rate to 5e-12 over 80 grid points, both in SNU with $\xi$ referred to the channel input.	✅ pass
test_limit_closes	Over pure loss the 8-PSK rate reaches 83.77%, 94.89%, 98.72% and 99.69% of the Gaussian rate at $V_A=0.2$, 0.05, 0.01 and 0.002.	✅ pass