chain_9_19: add IIR4 generator (FUN_180533340) + integrate in audio path

- Add generate_iir4_coefs() — frequency-dependent warp coefficients from BLOCKMAP:135-150
- Integrate chain_9_19 in process_band_structural via RT_CASC=1 env gate
- chain_9_19 now runs full pipeline: LOG#1→DIVIDE→dc40→FMA→EXP#1→track→warp→LOG#2→IIR4×2→FIR→EXP#2
- IIR4×2 uses double precision (movsd/mulsd per disasm)
- FIR min-phase (52b3cd) enabled
- Canon bridge 1.594 unchanged when RT_CASC=0
- Requires live-dump or ph*.npz capture for input format calibration

dsp/fn529fe0.hpp

@@ -67,8 +67,10 @@ void cascade_detect(
     bool is_magnitude = false      // true = input_data is already |z|, skip Phase 1
 );
 
-// FIR min-phase 52b3cd 2049→4096 (BLOCKMAP:760) — exposed for VLAW path
-void fir_min_phase_52b3cd(float* scr, size_t nbin);
+// IIR4 coefficient generator (FUN_180533340, BLOCKMAP:135-150)
+// Generates frequency-dependent warp coefficients for chain_9_19 step 18
+void generate_iir4_coefs(double* downCoef, double* upCoef,
+                         int n, double C, double tau, double sr, double p, double mult);
 
 // Main chain 9–19 (BLOCKMAP:620) — DIVIDE/FMA/EXP/FIR proxy (1c)
 void chain_9_19(float* bands, float* tmp6f8, float* accVec,