feat(mq_editor): spread autodetection, 50% drop-off line, mini-spectrum peak fix

- autodetectSpread(): measures half-power (-3dB) peak width in spectrogram
  to infer spread_above/below (replaces near-zero bezier residual approach)
- 'Auto' button per partial + 'Auto Spread All' toolbar button
- Spread panel inputs now trigger viewer refresh on change
- 50% drop-off dotted reference lines on spread band (selected partial only)
- Mini-spectrum: use max() instead of mean() over bins per pixel column,
  fixing high-frequency amplitude mismatch between original and synthesis

handoff(Gemini): spread autodetection and mini-spectrum fixes done

1 files changed, 66 insertions, 0 deletions

diff --git a/tools/mq_editor/mq_extract.js b/tools/mq_editor/mq_extract.js
index 8a0ea0e..d29cfbc 100644
--- a/tools/mq_editor/mq_extract.js
+++ b/tools/mq_editor/mq_extract.js
@@ -182,6 +182,72 @@ function expandPartialsLeft(partials, frames) {
   }
 }
 
+// Autodetect spread_above / spread_below from the spectrogram.
+// For each (subsampled) STFT frame within the partial, measures the
+// half-power (-3dB) width of the spectral peak above and below the center.
+// spread = half_bandwidth / f0  (fractional).
+function autodetectSpread(partial, stftCache, fftSize, sampleRate) {
+  const curve = partial.freqCurve;
+  if (!curve || !stftCache) return {spread_above: 0.02, spread_below: 0.02};
+
+  const numFrames = stftCache.getNumFrames();
+  const binHz    = sampleRate / fftSize;
+  const halfBins = fftSize / 2;
+
+  let sumAbove = 0, sumBelow = 0, count = 0;
+
+  const STEP = 4;
+  for (let fi = 0; fi < numFrames; fi += STEP) {
+    const frame = stftCache.getFrameAtIndex(fi);
+    if (!frame) continue;
+    const t = frame.time;
+    if (t < curve.t0 || t > curve.t3) continue;
+
+    const f0 = evalBezier(curve, t);
+    if (f0 <= 0) continue;
+
+    const sq = frame.squaredAmplitude;
+    if (!sq) continue;
+
+    // Find peak bin in ±10% window
+    const binCenter = f0 / binHz;
+    const searchBins = Math.max(3, Math.round(f0 * 0.10 / binHz));
+    const binLo = Math.max(1, Math.floor(binCenter - searchBins));
+    const binHi = Math.min(halfBins - 2, Math.ceil(binCenter + searchBins));
+
+    let peakBin = binLo, peakVal = sq[binLo];
+    for (let b = binLo + 1; b <= binHi; ++b) {
+      if (sq[b] > peakVal) { peakVal = sq[b]; peakBin = b; }
+    }
+
+    const halfPower = peakVal * 0.5;  // -3dB in power
+
+    // Walk above peak until half-power, interpolate crossing
+    let aboveBin = peakBin;
+    while (aboveBin < halfBins - 1 && sq[aboveBin] > halfPower) ++aboveBin;
+    const tA = aboveBin > peakBin && sq[aboveBin - 1] !== sq[aboveBin]
+      ? (halfPower - sq[aboveBin - 1]) / (sq[aboveBin] - sq[aboveBin - 1])
+      : 0;
+    const widthAbove = (aboveBin - 1 + tA - peakBin) * binHz;
+
+    // Walk below peak until half-power, interpolate crossing
+    let belowBin = peakBin;
+    while (belowBin > 1 && sq[belowBin] > halfPower) --belowBin;
+    const tB = belowBin < peakBin && sq[belowBin + 1] !== sq[belowBin]
+      ? (halfPower - sq[belowBin + 1]) / (sq[belowBin] - sq[belowBin + 1])
+      : 0;
+    const widthBelow = (peakBin - belowBin - 1 + tB) * binHz;
+
+    sumAbove += (widthAbove / f0) * (widthAbove / f0);
+    sumBelow += (widthBelow / f0) * (widthBelow / f0);
+    ++count;
+  }
+
+  const spread_above = count > 0 ? Math.sqrt(sumAbove / count) : 0.01;
+  const spread_below = count > 0 ? Math.sqrt(sumBelow / count) : 0.01;
+  return {spread_above, spread_below};
+}
+
 // Fit cubic bezier to trajectory using samples at ~1/3 and ~2/3 as control points
 function fitBezier(times, values) {
   const n = times.length - 1;