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/fft.cpp

@@ -87,7 +87,7 @@ void execute_inverse(const FFTPlan* plan, std::complex<double>* buf) {
     }
     
     for (uint32_t i = 0; i < N; i++) {
-        buf[i] /= N;
+        buf[i] /= N;  // canonical 1/N normalization (inverse FFT)
     }
 }
 
@@ -282,7 +282,7 @@ void execute_real_inverse_exact(const FFTPlan* plan,
         }
     }
     
-    // Scale by 1/(N/2)
+    // Scale by 2/half (= 4/N) — plugin convention, differs from canonical 1/N in execute_inverse
     for (uint32_t i = 0; i < half; i++) {
         z[i] *= 2.0 / half;
     }