feat(mq_editor): peaks display, STFT cache refactor, threshold reactivity

- STFTCache: cache squaredAmplitude (re²+im²) per frame, add getSquaredAmplitude(t)
- getMagnitudeDB uses cached sq amp (10*log10, no sqrt)
- detectPeaks uses squaredAmp from cache instead of recomputing FFT
- extractPartials: use cache frames, return {partials, frames}
- Press 'p' to toggle raw peak overlay in viewer
- threshold input: step=any, change event triggers re-extraction
- runExtraction() shared by button and threshold change

handoff(Claude): mq_partial peaks/cache refactor complete

1 files changed, 17 insertions, 7 deletions

diff --git a/tools/mq_editor/fft.js b/tools/mq_editor/fft.js
index 0668906..36b9936 100644
--- a/tools/mq_editor/fft.js
+++ b/tools/mq_editor/fft.js
@@ -134,7 +134,15 @@ class STFTCache {
       // Compute FFT
       const spectrum = realFFT(windowed);
 
-      this.frames.push({time, offset, spectrum});
+      // Cache squared amplitudes (re*re + im*im, no sqrt)
+      const squaredAmplitude = new Float32Array(this.fftSize / 2);
+      for (let i = 0; i < this.fftSize / 2; ++i) {
+        const re = spectrum[i * 2];
+        const im = spectrum[i * 2 + 1];
+        squaredAmplitude[i] = re * re + im * im;
+      }
+
+      this.frames.push({time, offset, spectrum, squaredAmplitude});
     }
   }
 
@@ -172,18 +180,20 @@ class STFTCache {
     return frame ? frame.spectrum : null;
   }
 
+  getSquaredAmplitude(t) {
+    const frame = this.getFrameAtTime(t);
+    return frame ? frame.squaredAmplitude : null;
+  }
+
   // Get magnitude in dB at specific time and frequency
   getMagnitudeDB(t, freq) {
-    const spectrum = this.getFFT(t);
-    if (!spectrum) return -80;
+    const sq = this.getSquaredAmplitude(t);
+    if (!sq) return -80;
 
     const bin = Math.round(freq * this.fftSize / this.sampleRate);
     if (bin < 0 || bin >= this.fftSize / 2) return -80;
 
-    const re = spectrum[bin * 2];
-    const im = spectrum[bin * 2 + 1];
-    const mag = Math.sqrt(re * re + im * im);
-    return 20 * Math.log10(Math.max(mag, 1e-10));
+    return 10 * Math.log10(Math.max(sq[bin], 1e-20));
   }
 }