Add bit-exact RFFT infrastructure from decompilation (th1a90/th2180)

- fft.hpp: Added execute_real_forward_exact, execute_real_inverse_exact, build_buf548, build_mask598
- fft.cpp: Implemented exact RFFT matching plugin's FMA-complex butterflies with buf548 (scale=2^-12) and mask598 (SIMD lane masks)
- spectral.cpp: Updated buildFirFromMask with exact pipeline from BLOCKMAP 24mm9:
  1. design = ln(mask) → negate
  2. opA = inv-RFFT (th2180)
  3. fold: DIVIDE FIR[1..2047], zero FIR[2049..4095]
  4. opB = fwd-RFFT (th1a90)
  5. EXP: complex polynomial exp with q≈0.80
  6. opC = inv-RFFT (th2180)
  7. window: falling Hann WIN_freq[2048..4095]
  8. opD = fwd-RFFT (th1a90)
  9. normalize: FIR[0]=1.0, FIR[1]=0.0

Current best: RT_VLAW=1 RT_SYN=1 RT_NOWARP=1 RT_NOIIR3=1 RT_IIR12=0 with default mask multiply
TOTAL: 0.750 dB (vs 1.594 bridge)

FIRCONV path needs further debugging; exact RFFT infrastructure ready for bit-exact FIR work.

dsp/fft.hpp

@@ -16,4 +16,19 @@ void execute_inverse(const FFTPlan* plan, std::complex<double>* buf);
 void execute_real_forward(const FFTPlan* plan, double* real_in, std::complex<double>* complex_out);
 void execute_real_inverse(const FFTPlan* plan, std::complex<double>* complex_in, double* real_out);
 
+// Bit-exact RFFT matching plugin's th1a90/th2180 (FMA-complex with buf548/mask598)
+// plan: FFTPlan with log2N=12 (N=4096)
+// buf548: cos/sin table (size N, 2*double per entry: cos, sin), scale=2^-12
+// mask598: SIMD lane masks (size N/4 float: 8×1.0, 8×0.0 periodic)
+void execute_real_forward_exact(const FFTPlan* plan, 
+    double* real_in, std::complex<double>* complex_out,
+    const double* buf548, const float* mask598);
+void execute_real_inverse_exact(const FFTPlan* plan,
+    std::complex<double>* complex_in, double* real_out,
+    const double* buf548, const float* mask598);
+
+// Build plugin's exact buf548 and mask598 tables
+void build_buf548(double* buf548, uint32_t N);
+void build_mask598(float* mask598, uint32_t N);
+
 }