Optimize DebugPanel drawing/painting (spectrogram + waveform)

[ysdede]

Context

• LayeredBufferVisualizer runs a continuous rAF loop (~30fps) and draws a full spectrogram using putImageData every 100ms.
• drawSpectrogramToCanvas touches every pixel (width * height), then uploads to canvas each time.

Evidence

• File: src/components/LayeredBufferVisualizer.tsx (loop + putImageData + per-frame waveform draw)
• Frame jank present in trace (max interval 80.1ms).

Hypothesis

• DebugPanel open increases paint time and GC due to large ImageData uploads and per-frame waveform sampling.

Actions

• Only redraw spectrogram when new features are available; skip putImageData if unchanged.
• Reduce resolution (downsample width/height) or render to OffscreenCanvas + ImageBitmap transfer.
• Pause the loop when not recording or when panel is not visible.

Acceptance

• With DebugPanel open, no additional long tasks >50ms vs panel closed.
• CPU time for DebugPanel rendering reduced by at least 30%.

[ysdede]

Performance Round 2 work landed in PR #143. This issue is partially addressed (debug panel off-path hardening and no post-unmount visualizer work), but the full acceptance criteria here (measured >=30% render CPU reduction and no long tasks delta) are not fully claimed yet.