feat(audio): use VHQ settings for SoXR resampling

lib/services/ffmpeg_service.dart

@@ -595,7 +595,7 @@ class FFmpegService {
     if (!hasSampleRate && !hasSampleFormat && !processing.hasDither) return;
 
     final options = <String>[
-      'resampler=${processing.normalizedResampler}',
+      ...losslessResamplerFilterOptions(processing),
       if (hasSampleRate) 'osr=$targetSampleRate',
       if (hasSampleFormat) 'osf=${outputSampleFormat.trim()}',
       if (processing.hasDither) 'dither_method=${processing.normalizedDither}',

lib/utils/audio_conversion_utils.dart

@@ -27,6 +27,9 @@ const List<String> losslessDitherOptions = [
 
 const List<String> losslessResamplerOptions = ['swr', 'soxr'];
 
+const int soxrVhqPrecisionBits = 28;
+const double soxrPassbandEndRatio = 0.95;
+
 class LosslessConversionProcessing {
   final String dither;
   final String resampler;
@@ -49,6 +52,19 @@ class LosslessConversionProcessing {
   bool get hasDither => normalizedDither != 'none';
 }
 
+List<String> losslessResamplerFilterOptions(
+  LosslessConversionProcessing processing,
+) {
+  final resampler = processing.normalizedResampler;
+  return [
+    'resampler=$resampler',
+    if (resampler == 'soxr') ...[
+      'precision=$soxrVhqPrecisionBits',
+      'cutoff=$soxrPassbandEndRatio',
+    ],
+  ];
+}
+
 List<int> availableLosslessBitDepthOptions(int? sourceBitDepth) {
   if (sourceBitDepth == null || sourceBitDepth <= 0) {
     return losslessConversionBitDepthOptions;

test/audio_conversion_utils_test.dart

@@ -44,6 +44,18 @@ void main() {
       expect(swrHighPass.normalizedDither, 'triangular_hp');
       expect(swrHighPass.hasDither, isTrue);
     });
+
+    test('uses VHQ and a 95 percent passband only for SoXR', () {
+      const soxr = LosslessConversionProcessing(resampler: 'soxr');
+      const swr = LosslessConversionProcessing(resampler: 'swr');
+
+      expect(losslessResamplerFilterOptions(soxr), [
+        'resampler=soxr',
+        'precision=28',
+        'cutoff=0.95',
+      ]);
+      expect(losslessResamplerFilterOptions(swr), ['resampler=swr']);
+    });
   });
 
   group('kept conversion output identity', () {