feat(mq_editor): Complete Phase 2 - JS synthesizer with STFT cache

Phase 2 - JS Synthesizer:
- Created mq_synth.js with replica oscillator bank
- Bezier curve evaluation (cubic De Casteljau algorithm)
- Replica synthesis: frequency spread, amplitude decay, phase jitter
- PCM buffer generation from extracted MQ partials
- Normalization to prevent clipping
- Key '1' plays synthesized audio, key '2' plays original
- Playback comparison with animated playhead

STFT Cache Optimization:
- Created STFTCache class in fft.js for pre-computed windowed FFT frames
- Clean interface: getFFT(t), getMagnitudeDB(t, freq), setHopSize()
- Pre-computes all frames on WAV load (eliminates redundant FFT calls)
- Dynamic cache update when hop size changes
- Shared across spectrogram, tooltip, and mini-spectrum viewer
- Significant performance improvement

Mini-Spectrum Viewer:
- Bottom-right overlay (200x100) matching spectral_editor style
- Real-time FFT display at playhead or mouse position
- 100-bar visualization with cyan-to-yellow gradient
- Updates during playback or mouse hover

Files:
- tools/mq_editor/mq_synth.js (new)
- tools/mq_editor/fft.js (STFTCache class added)
- tools/mq_editor/index.html (synthesis playback, cache integration)
- tools/mq_editor/viewer.js (cache-based rendering, spectrum viewer)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

1 files changed, 85 insertions, 6 deletions

diff --git a/tools/mq_editor/index.html b/tools/mq_editor/index.html
index 1a07b61..84abd1c 100644
--- a/tools/mq_editor/index.html
+++ b/tools/mq_editor/index.html
@@ -86,7 +86,14 @@
     </div>
   </div>
 
-  <canvas id="canvas" width="1400" height="600"></canvas>
+  <div style="position: relative;">
+    <canvas id="canvas" width="1400" height="600"></canvas>
+
+    <!-- Mini spectrum viewer (bottom-right overlay) -->
+    <div id="spectrumViewer" style="position: absolute; bottom: 10px; right: 10px; width: 200px; height: 100px; background: rgba(30, 30, 30, 0.9); border: 1px solid #555; border-radius: 3px; pointer-events: none;">
+      <canvas id="spectrumCanvas" width="200" height="100"></canvas>
+    </div>
+  </div>
 
   <div id="tooltip" style="position: fixed; display: none; background: #2a2a2a; padding: 4px 8px; border: 1px solid #555; border-radius: 3px; pointer-events: none; font-size: 11px; z-index: 1000;"></div>
 
@@ -94,12 +101,15 @@
 
   <script src="fft.js"></script>
   <script src="mq_extract.js"></script>
+  <script src="mq_synth.js"></script>
   <script src="viewer.js"></script>
   <script>
     let audioBuffer = null;
     let viewer = null;
     let audioContext = null;
     let currentSource = null;
+    let extractedPartials = null;
+    let stftCache = null;
 
     const wavFile = document.getElementById('wavFile');
     const extractBtn = document.getElementById('extractBtn');
@@ -133,16 +143,36 @@
         initAudioContext();
         extractBtn.disabled = false;
         playBtn.disabled = false;
-        setStatus(`Loaded: ${audioBuffer.duration.toFixed(2)}s, ${audioBuffer.sampleRate}Hz, ${audioBuffer.numberOfChannels}ch`, 'info');
+        setStatus('Computing STFT cache...', 'info');
 
-        // Create viewer
-        viewer = new SpectrogramViewer(canvas, audioBuffer);
+        // Compute STFT cache
+        setTimeout(() => {
+          const signal = audioBuffer.getChannelData(0);
+          stftCache = new STFTCache(signal, audioBuffer.sampleRate, fftSize, parseInt(hopSize.value));
+
+          setStatus(`Loaded: ${audioBuffer.duration.toFixed(2)}s, ${audioBuffer.sampleRate}Hz, ${audioBuffer.numberOfChannels}ch (${stftCache.getNumFrames()} frames cached)`, 'info');
+
+          // Create viewer with cache
+          viewer = new SpectrogramViewer(canvas, audioBuffer, stftCache);
+        }, 10);
       } catch (err) {
         setStatus('Error loading WAV: ' + err.message, 'error');
         console.error(err);
       }
     });
 
+    // Update cache when hop size changes
+    hopSize.addEventListener('change', () => {
+      if (stftCache) {
+        setStatus('Updating STFT cache...', 'info');
+        setTimeout(() => {
+          stftCache.setHopSize(parseInt(hopSize.value));
+          setStatus(`Cache updated (${stftCache.getNumFrames()} frames)`, 'info');
+          if (viewer) viewer.render();
+        }, 10);
+      }
+    });
+
     // Extract partials
     extractBtn.addEventListener('click', () => {
       if (!audioBuffer) return;
@@ -161,6 +191,7 @@
 
           const partials = extractPartials(audioBuffer, params);
 
+          extractedPartials = partials;
           setStatus(`Extracted ${partials.length} partials`, 'info');
 
           // Update viewer
@@ -235,12 +266,60 @@
       status.className = type;
     }
 
+    // Play synthesized audio
+    function playSynthesized() {
+      if (!extractedPartials || extractedPartials.length === 0) {
+        setStatus('No partials extracted yet', 'warn');
+        return;
+      }
+      if (!audioBuffer || !audioContext) return;
+
+      stopAudio();
+
+      setStatus('Synthesizing...', 'info');
+
+      // Synthesize PCM from partials
+      const sampleRate = audioBuffer.sampleRate;
+      const duration = audioBuffer.duration;
+      const pcm = synthesizeMQ(extractedPartials, sampleRate, duration);
+
+      // Create audio buffer
+      const synthBuffer = audioContext.createBuffer(1, pcm.length, sampleRate);
+      synthBuffer.getChannelData(0).set(pcm);
+
+      const startTime = audioContext.currentTime;
+      currentSource = audioContext.createBufferSource();
+      currentSource.buffer = synthBuffer;
+      currentSource.connect(audioContext.destination);
+      currentSource.start();
+
+      currentSource.onended = () => {
+        currentSource = null;
+        playBtn.disabled = false;
+        stopBtn.disabled = true;
+        viewer.setPlayheadTime(-1);
+        setStatus('Stopped', 'info');
+      };
+
+      playBtn.disabled = true;
+      stopBtn.disabled = false;
+      setStatus('Playing synthesized...', 'info');
+
+      // Animate playhead
+      function updatePlayhead() {
+        if (!currentSource) return;
+        const elapsed = audioContext.currentTime - startTime;
+        viewer.setPlayheadTime(elapsed);
+        requestAnimationFrame(updatePlayhead);
+      }
+      updatePlayhead();
+    }
+
     // Keyboard shortcuts
     document.addEventListener('keydown', (e) => {
       if (e.code === 'Digit1') {
         e.preventDefault();
-        // TODO: Play synthesized (Phase 2)
-        setStatus('Synthesized playback not yet implemented', 'warn');
+        playSynthesized();
       } else if (e.code === 'Digit2') {
         e.preventDefault();
         if (!playBtn.disabled) {