k-mapping: dedup to k_mapping_factor helper, add RT_KMAP_FC

- Extract k_sens (linear 6->0.44, exp 12->22.0) + k_q log-interp 0.5->1.0 2.0->0.403 into helper
- Replace duplicate blocks for lvl_in/raw_level with single call
- Add k_fc=1.0 + opt-in RT_KMAP_FC via W_eq (gated)
- Gates: bridge 1.594, structural 2.689 (RT_KMAP=1 same, no regression)

dsp/framed_model.cpp

@@ -57,6 +57,32 @@ static bool is_internal_grid(size_t nfft, float sample_rate) {
     return nfft == 4096 && std::abs(sample_rate - 48000.0f) < 1.0f;
 }
 
+static double k_mapping_factor(float fc, float q, float sens) {
+    // k(sens,q,fc) = k_sens * k_q * k_fc  (NOTES 24x/24dd/24ee)
+    // Fitted: k_sens 6→0.44 12→1.0 18→5.37 24→22.0 (exp after 12, linear before)
+    //         k_q 0.5→1.0 2.0→0.403 (log interp, q no effect above 2 per 24kk)
+    //         k_fc 1.0 for now (fc via W_eq weak, keep 1.0 gated RT_KMAP_FC)
+    double k_sens;
+    if (sens < 12) k_sens = 0.44 + (sens - 6.0) * (0.56 / 6.0);
+    else if (sens == 12) k_sens = 1.0;
+    else if (sens < 24) k_sens = std::exp((sens - 12.0) * std::log(22.0) / 12.0);
+    else k_sens = 22.0;
+    double k_q;
+    if (q >= 2.0) k_q = 0.403;
+    else if (q <= 0.5) k_q = 1.0;
+    else {
+        double t = (std::log(q) - std::log(0.5)) / (std::log(2.0) - std::log(0.5));
+        k_q = 1.0 + t * (0.403 - 1.0);
+    }
+    double k_fc = 1.0;
+    if (getenv("RT_KMAP_FC")) {
+        // opt-in fc factor via W_eq magnitude at fc (simple H=1 at fc)
+        double w_fc = std::pow(10.0, (sens * 0.3 / 12.0) / 20.0);
+        k_fc = 1.0 / w_fc; // naive, gated
+    }
+    return k_sens * k_q * k_fc;
+}
+
 static void process_band_structural(
     const float* am,
     const float* res,
@@ -117,34 +143,10 @@ static void process_band_structural(
         lvl_in[k] = static_cast<float>(static_cast<double>(am[k]) / res_k * scale_factor_x);
     }
     // k-mapping per NOTES 24x/24dd/24ee: lvl_impl = lvl_ours / k(sens,q,fc)
-    // k = k_sens(sens) * k_q(q) * k_fc(fc) ; default OFF (canon), opt-in RT_KMAP=1
+    // default OFF (canon), opt-in RT_KMAP=1 (helper k_mapping_factor)
-    // Fitted from table 24x: k_sens 6→0.44, 12→1.0, 18→5.37, 24→22.0 ; k_q 0.5→1.0, 2.0→0.403
+    static const int kmap_on = getenv("RT_KMAP") ? atoi(getenv("RT_KMAP")) : 0;
-    static const int kmap_on = []{ const char* e=getenv("RT_KMAP"); return e ? atoi(e) : 0; }();
     if (kmap_on) {
-        double k_sens;
+        double k_tot = k_mapping_factor(band.fc, band.q, band.sens);
-        if (band.sens < 12) {
-            // 6→0.44, 12→1.0 linear
-            k_sens = 0.44 + (band.sens - 6.0) * (0.56 / 6.0);
-        } else if (band.sens == 12) {
-            k_sens = 1.0;
-        } else if (band.sens < 24) {
-            // 12→1.0, 24→22.0 exponential
-            k_sens = std::exp((band.sens - 12.0) * std::log(22.0) / 12.0);
-        } else {
-            k_sens = 22.0;
-        }
-        double k_q;
-        if (band.q >= 2.0) k_q = 0.403;
-        else if (band.q <= 0.5) k_q = 1.0;
-        else {
-            // interpolate log q 0.5→2.0 : 1.0→0.403
-            double t = (std::log(band.q) - std::log(0.5)) / (std::log(2.0) - std::log(0.5));
-            k_q = 1.0 + t * (0.403 - 1.0);
-        }
-        double k_fc = 1.0;
-        // fc 500→1.0, 1000→~1.4 per 24w-2 (1.15@500 vs 1.62@1000) -> k_fc 1.0→0.85?
-        // Keep 1.0 for now; fc effect is weak vs sens/q.
-        double k_tot = k_sens * k_q * k_fc;
         if (k_tot > 1e-9) {
             for (size_t k = 0; k < nbin; k++) lvl_in[k] = static_cast<float>(lvl_in[k] / k_tot);
         }
@@ -202,19 +204,7 @@ static void process_band_structural(
         raw_level[k] = static_cast<float>(static_cast<double>(am[k]) / res_k * scale_factor_x);
     }
     if (kmap_on) {
-        double k_sens;
+        double k_tot2 = k_mapping_factor(band.fc, band.q, band.sens);
-        if (band.sens < 12) k_sens = 0.44 + (band.sens - 6.0) * (0.56 / 6.0);
-        else if (band.sens == 12) k_sens = 1.0;
-        else if (band.sens < 24) k_sens = std::exp((band.sens - 12.0) * std::log(22.0) / 12.0);
-        else k_sens = 22.0;
-        double k_q;
-        if (band.q >= 2.0) k_q = 0.403;
-        else if (band.q <= 0.5) k_q = 1.0;
-        else {
-            double t = (std::log(band.q) - std::log(0.5)) / (std::log(2.0) - std::log(0.5));
-            k_q = 1.0 + t * (0.403 - 1.0);
-        }
-        double k_tot2 = k_sens * k_q;
         if (k_tot2 > 1e-9) for (size_t k = 0; k < nbin; k++) raw_level[k] = static_cast<float>(raw_level[k] / k_tot2);
     }
     static const int eq_on2 = getenv("RT_EQ") ? atoi(getenv("RT_EQ")) : 1;