feat: add CUDA FIR preparation path

internal/demod/gpudemod/gpudemod.go

@@ -229,8 +229,12 @@ func (e *Engine) Demod(iq []complex64, offsetHz float64, bw float64, mode DemodT
 	if len(taps) == 0 {
 		base := dsp.LowpassFIR(cutoff, e.sampleRate, 101)
 		taps = append(make([]float32, 0, len(base)), base...)
+		e.SetFIR(taps)
+	}
+	filtered, ok := e.tryCUDAFIR(shifted, len(taps))
+	if !ok {
+		filtered = dsp.ApplyFIR(shifted, taps)
 	}
-	filtered := dsp.ApplyFIR(shifted, taps)
 	decim := int(math.Round(float64(e.sampleRate) / float64(outRate)))
 	if decim < 1 {
 		decim = 1
@@ -285,3 +289,35 @@ func (e *Engine) Close() {
 	e.firTaps = nil
 	e.cudaReady = false
 }
+odLSB:
+		return demod.LSB{}.Demod(dec, inputRate), inputRate, nil
+	case DemodCW:
+		return demod.CW{}.Demod(dec, inputRate), inputRate, nil
+	default:
+		return nil, 0, errors.New("unsupported demod type")
+	}
+}
+
+func (e *Engine) Close() {
+	if e == nil {
+		return
+	}
+	if e.dIQIn != nil {
+		_ = C.gpud_cuda_free(unsafe.Pointer(e.dIQIn))
+		e.dIQIn = nil
+	}
+	if e.dShifted != nil {
+		_ = C.gpud_cuda_free(unsafe.Pointer(e.dShifted))
+		e.dShifted = nil
+	}
+	if e.dDecimated != nil {
+		_ = C.gpud_cuda_free(unsafe.Pointer(e.dDecimated))
+		e.dDecimated = nil
+	}
+	if e.dAudio != nil {
+		_ = C.gpud_cuda_free(unsafe.Pointer(e.dAudio))
+		e.dAudio = nil
+	}
+	e.firTaps = nil
+	e.cudaReady = false
+}

internal/demod/gpudemod/kernels.cu

@@ -72,6 +72,49 @@ extern "C" __global__ void gpud_decimate_kernel(
     out[idx] = in[idx * factor];
 }
 
+extern "C" __constant__ float gpud_fir_taps[256];
+
+extern "C" __global__ void gpud_fir_kernel(
+    const float2* __restrict__ in,
+    float2* __restrict__ out,
+    int n,
+    int num_taps
+) {
+    int idx = blockIdx.x * blockDim.x + threadIdx.x;
+    if (idx >= n) return;
+
+    float acc_r = 0.0f;
+    float acc_i = 0.0f;
+    for (int k = 0; k < num_taps; ++k) {
+        int src = idx - k;
+        if (src < 0) break;
+        float2 v = in[src];
+        float t = gpud_fir_taps[k];
+        acc_r += v.x * t;
+        acc_i += v.y * t;
+    }
+    out[idx] = make_float2(acc_r, acc_i);
+}
+
+extern "C" int gpud_upload_fir_taps_cuda(const float* taps, int n) {
+    if (!taps || n <= 0 || n > 256) return -1;
+    cudaError_t err = cudaMemcpyToSymbol(gpud_fir_taps, taps, (size_t)n * sizeof(float));
+    return (int)err;
+}
+
+extern "C" int gpud_launch_fir_cuda(
+    const float2* in,
+    float2* out,
+    int n,
+    int num_taps
+) {
+    if (n <= 0 || num_taps <= 0 || num_taps > 256) return 0;
+    const int block = 256;
+    const int grid = (n + block - 1) / block;
+    gpud_fir_kernel<<<grid, block>>>(in, out, n, num_taps);
+    return (int)cudaGetLastError();
+}
+
 extern "C" int gpud_launch_decimate_cuda(
     const float2* in,
     float2* out,