fix(analysis): reject false spectral rolloff cutoffs

lib/widgets/audio_analysis_models.dart

@@ -3,7 +3,7 @@ part of 'audio_analysis_widget.dart';
 // Analysis result models and per-run parameter records.
 
 class AudioAnalysisData {
-  static const cacheVersion = 10;
+  static const cacheVersion = 12;
 
   final String filePath;
   final int fileSize;

lib/widgets/audio_analysis_widget.dart

@@ -381,6 +381,24 @@ double? estimateEffectiveSpectralCutoffHz({
   }
 
   final hzPerRow = maxFrequencyHz / height;
+
+  // Reject narrow horizontal lines before looking for a bandwidth edge. A
+  // median over roughly 500 Hz preserves a broadband step while removing
+  // pilots, tones, and isolated noisy bins that occupy only a small fraction
+  // of the surrounding band.
+  final lineRejectionRadius = math.max(1, (250 / hzPerRow).ceil());
+  final broadbandProfile = Float64List(height);
+  for (var index = 0; index < height; index++) {
+    broadbandProfile[index] = _spectralMedian(
+      profile,
+      index - lineRejectionRadius,
+      index + lineRejectionRadius + 1,
+    );
+  }
+
+  // Apply only light smoothing after the robust frequency aggregation. This
+  // reduces row quantization without letting a high-amplitude line smear into
+  // enough adjacent bins to resemble broadband support.
   final smoothingRadius = math.max(1, (50 / hzPerRow).ceil());
   final smoothed = Float64List(height);
   var running = 0.0;
@@ -391,10 +409,10 @@ double? estimateEffectiveSpectralCutoffHz({
     final desiredEnd = math.min(height - 1, index + smoothingRadius);
     while (windowEnd < desiredEnd) {
       windowEnd++;
-      running += profile[windowEnd];
+      running += broadbandProfile[windowEnd];
     }
     while (windowStart < desiredStart) {
-      running -= profile[windowStart];
+      running -= broadbandProfile[windowStart];
       windowStart++;
     }
     smoothed[index] = running / (windowEnd - windowStart + 1);
@@ -437,7 +455,11 @@ double? estimateEffectiveSpectralCutoffHz({
     return bDrop.compareTo(aDrop);
   });
 
-  final minimumDrop = math.max(12.0, dynamicSpan * 0.18);
+  // A sharp, frequency-contiguous edge remains meaningful at lower contrast
+  // than an arbitrary bright bin. Keep the threshold relative to the measured
+  // spectral span, but do not require the old fixed 12-byte difference that
+  // caused elevated noise floors to be classified as full-band.
+  final minimumDrop = math.max(6.0, dynamicSpan * 0.18);
   final gapRows = math.max(1, (100 / hzPerRow).ceil());
   final supportRows = math.max(3, (1200 / hzPerRow).ceil());
   final stableTailSpread = math.max(4.0, dynamicSpan * 0.06);
@@ -478,7 +500,11 @@ double? estimateEffectiveSpectralCutoffHz({
     (height * 0.90).floor(),
     math.max(1, (height * 0.98).floor()),
   );
-  final activeMargin = math.max(10.0, dynamicSpan * 0.20);
+  // A gradual cutoff needs stronger contrast than a sharp, contiguous edge.
+  // Otherwise an ordinary full-band spectral tilt can eventually cross the
+  // high-frequency floor by a few grayscale steps and create an arbitrary
+  // cutoff. Require a material transition into the stable tail instead.
+  final activeMargin = math.max(12.0, dynamicSpan * 0.25);
   final activeThreshold = tailReferenceLevel + activeMargin;
   for (var edgeIndex = searchEnd - 1; edgeIndex >= searchStart; edgeIndex--) {
     final belowEnd = edgeIndex - gapRows;
@@ -519,8 +545,13 @@ double? estimateEffectiveSpectralCutoffHz({
     math.max(1, (height * 0.90).floor()),
   );
   final topBandNearPeak = topBandLevel >= highLevel - minimumDrop;
+  // A real low-contrast full-band slope can change by less than the tail
+  // stability tolerance. Preserve it when the upper octave still trends
+  // downward by at least one grayscale step; a low-pass noise plateau remains
+  // flat here and must already have passed the validated edge checks above.
+  final upperBandSlope = lowerTopBandLevel - topBandLevel;
   final topBandStillSloping =
-      lowerTopBandLevel - topBandLevel > stableTailSpread;
+      upperBandSlope >= math.max(1.0, dynamicSpan * 0.05);
   if (basebandLevel >= 24 &&
       topBandLevel >= 24 &&
       (topBandNearPeak || topBandStillSloping)) {

test/audio_analysis_spectrogram_test.dart

@@ -22,6 +22,12 @@ void main() {
     });
   });
 
+  group('audio analysis cache', () {
+    test('invalidates results from the previous cutoff estimator', () {
+      expect(AudioAnalysisData.cacheVersion, 12);
+    });
+  });
+
   group('audio level analysis', () {
     test('reads peak and RMS from the final astats overall summary', () {
       const logs = '''
@@ -275,6 +281,66 @@ lavfi.r128.true_peak=0.907
       expect(cutoff!, inInclusiveRange(15300, 16300));
     });
 
+    test(
+      'finds a 15 kHz edge below elevated noise and a persistent 18.7 kHz line',
+      () {
+        const cdNyquist = 22050.0;
+        final intensity = _blankIntensity(width, height, value: 30);
+        _paintFrequencyBand(
+          intensity,
+          width: width,
+          height: height,
+          maxFrequencyHz: cdNyquist,
+          lowHz: 0,
+          highHz: 15000,
+          intensity: 39,
+        );
+        _paintFrequencyBand(
+          intensity,
+          width: width,
+          height: height,
+          maxFrequencyHz: cdNyquist,
+          lowHz: 18600,
+          highHz: 18800,
+          intensity: 220,
+        );
+
+        final cutoff = estimateEffectiveSpectralCutoffHz(
+          intensity: intensity,
+          width: width,
+          height: height,
+          maxFrequencyHz: cdNyquist,
+        );
+
+        expect(cutoff, isNotNull);
+        expect(cutoff!, inInclusiveRange(14500, 15500));
+      },
+    );
+
+    test('retains a genuine broadband cutoff around 18.7 kHz', () {
+      const cdNyquist = 22050.0;
+      final intensity = _blankIntensity(width, height, value: 18);
+      _paintFrequencyBand(
+        intensity,
+        width: width,
+        height: height,
+        maxFrequencyHz: cdNyquist,
+        lowHz: 0,
+        highHz: 18700,
+        intensity: 100,
+      );
+
+      final cutoff = estimateEffectiveSpectralCutoffHz(
+        intensity: intensity,
+        width: width,
+        height: height,
+        maxFrequencyHz: cdNyquist,
+      );
+
+      expect(cutoff, isNotNull);
+      expect(cutoff!, inInclusiveRange(18200, 19200));
+    });
+
     test('ignores a tonal drop before a gradual 22 kHz bandwidth limit', () {
       const hiresNyquist = 48000.0;
       final intensity = _blankIntensity(width, height, value: 46);
@@ -379,6 +445,65 @@ lavfi.r128.true_peak=0.907
       },
     );
 
+    test('reports Nyquist for low-contrast full-band spectral tilt', () {
+      const cdNyquist = 22050.0;
+      final intensity = _blankIntensity(width, height);
+      _paintNaturalSpectralTilt(
+        intensity,
+        width: width,
+        height: height,
+        lowFrequencyIntensity: 39,
+        nyquistIntensity: 30,
+      );
+
+      final cutoff = estimateEffectiveSpectralCutoffHz(
+        intensity: intensity,
+        width: width,
+        height: height,
+        maxFrequencyHz: cdNyquist,
+      );
+
+      expect(cutoff, cdNyquist);
+    });
+
+    test('reports Nyquist for an extended gentle high-frequency rolloff', () {
+      const cdNyquist = 22050.0;
+      final intensity = _blankIntensity(width, height);
+      const segments = <(double, double, int, int)>[
+        (0, 4000, 113, 104),
+        (4000, 6000, 104, 96),
+        (6000, 15000, 96, 91),
+        (15000, 16000, 91, 87),
+        (16000, 17000, 87, 84),
+        (17000, 18500, 84, 82),
+        (18500, 19000, 82, 78),
+        (19000, 20000, 78, 73),
+        (20000, 21000, 73, 70),
+        (21000, cdNyquist, 70, 69),
+      ];
+      for (final segment in segments) {
+        _paintFrequencySlope(
+          intensity,
+          width: width,
+          height: height,
+          maxFrequencyHz: cdNyquist,
+          lowHz: segment.$1,
+          highHz: segment.$2,
+          lowIntensity: segment.$3,
+          highIntensity: segment.$4,
+        );
+      }
+
+      final cutoff = estimateEffectiveSpectralCutoffHz(
+        intensity: intensity,
+        width: width,
+        height: height,
+        maxFrequencyHz: cdNyquist,
+      );
+
+      expect(cutoff, cdNyquist);
+    });
+
     test('does not report an isolated line as a broadband cutoff', () {
       final intensity = _blankIntensity(width, height);
       _paintFrequencyBand(