pub fn noise_variance(eb_n0_db: f32, code_rate: f32) -> f32
Compute noise variance σ² from Eb/N₀ (in dB) and code rate.
For BPSK: σ² = 1 / (2 · rate · 10^(eb_n0_db/10)) For QPSK: same formula (QPSK has same BER vs Eb/N₀ as BPSK).