audit fixes: dead conditional, DBG_CASC gate, dup loop, fft comments, blend doc

- framed_model: remove `if (pool_w > 0 && !lut_off == false) {}` (empty body)
- framed_model: gate DBG_CASC fprintf behind RT_DBG_CASC (was per-frame spam)
- framed_model: document f6f8 blend 0.8 (decomp 0x5406f8, xmm10 @1824c3e28)
- framed_model: assert(spectrum != nullptr) in processFrame
- spectral: remove duplicate upper-half zero loop in buildFirFromMask
- fft: comment scaling difference (1/N canonical vs 2/half plugin convention)
- Guard: bridge corpus --compare d=+0.000 (exact parity)

dsp/spectral.cpp

@@ -207,9 +207,6 @@ void SpectralProcessor::buildFirFromMask(const float* mask, std::complex<double>
     for (size_t i = half + 1; i < nfft; i++) {
         time_domain[i] = 0.0;  // xmm9 = 0 zeros upper half
     }
-    for (size_t i = half + 1; i < nfft; i++) {
-        time_domain[i] = 0.0;
-    }
 
     // Step 4: opB = fwd-RFFT (th1a90): time_domain (real) → complex
     std::vector<std::complex<double>> freq_domain(half + 1);