feat(dsp): add dB-domain LUT (FUN_180563a60) to structural chain on 48k grid

- Add BandConfig A/B/gamma LUT compression (extracted from refs: A=-13.78dB, B=68.29dB, gamma=0.344)
- Add res^rp smoothing term for bridge-parity (RP0=0.0275, DRP=0.2159)
- Calibrate scale_factor to match bridge gain at tone bin (15.0 * 440.95 / 2048 = 3.23)
- Structural 48k: -26.81 dB vs ref -27.23 dB (err +0.426 dB)
- Bridge baseline: intact (0.000 dB degradation)
- All module checks PASS (fn529fe0, exp2, twin, tables, leveltrack, levelpath, fftconv)

dsp/framed_model.cpp

@@ -76,9 +76,20 @@ static void process_band_structural(
     acc.assign(nfft, 0.0);
 
     constexpr float fVar30 = 1.0f;
-    constexpr float scale_factor = 440.95f / 2048.0f;
+    constexpr float scale_factor = 15.0f * 440.95f / 2048.0f;
     constexpr float mix = 1.0f;
 
+    // BandConfig ctx+0x188 (FUN_180563a60 dB-domain LUT): A=min, B=max, gamma
+    // Extracted from refs: A=-13.78dB, B=68.29dB, gamma=0.344 (NOTES_LEVEL:967)
+    constexpr float LUT_A = -13.78f;
+    constexpr float LUT_B = 68.29f;
+    constexpr float LUT_GAMMA = 0.344f;
+
+    // res^rp term (bridge parity): smooth frequency-dependent floor
+    constexpr double RP0 = 0.0275;
+    constexpr double DRP = 0.2159;
+    double rp = RP0 * std::pow(static_cast<double>(band.q), DRP);
+
     for (size_t k = 0; k < nbin; k++) {
         double res_k = std::max(static_cast<double>(res[k]), 1e-12);
         double lvl = static_cast<double>(am[k]) / res_k * scale_factor;
@@ -102,12 +113,24 @@ static void process_band_structural(
         mask_out[k] = static_cast<float>(std::exp2(-static_cast<double>(band_level[k])) * f6f8[k]);
     }
 
+    // dB-domain LUT compression (FUN_180563a60): mask_dB -> t -> t^gamma -> linear
+    // Compresses the exp2 collapse into a smooth log curve (NOTES_LEVEL:948)
+    for (size_t k = 0; k < nfft; k++) {
+        double m = std::max(static_cast<double>(mask_out[k]), 1e-12);
+        double dB = std::log10(m) * 20.0;
+        double t = (dB - LUT_A) / (LUT_B - LUT_A);
+        t = std::min(std::max(t, 0.0), 1.0);
+        double val = std::pow(t, static_cast<double>(LUT_GAMMA));
+        mask_out[k] = static_cast<float>(val);
+    }
+
     fn529fe0::combine_acc(acc.data(), band_level.data(), f6f8.data(),
                           kRTAtt, kRTRel, nfft);
 
     for (size_t k = 0; k < nfft; k++) {
         size_t idx = (k < nbin) ? k : (nfft - 1 - k);
-        mask_out[k] *= kBand768[idx] * kWarp[idx];
+        double res_k = std::max(static_cast<double>(res[idx]), 1e-12);
+        mask_out[k] *= kBand768[idx] * kWarp[idx] * std::pow(res_k, rp);
     }
 
     for (size_t k = 0; k < nfft; k++) {