Perf(debug): measure and trim remaining DebugPanel paint cost

[ysdede]

Summary

A lot of the first-round DebugPanel work is already in place on master:

• LayeredBufferVisualizer.tsx caches canvas dimensions via ResizeObserver.
• DPR changes are handled outside the hot draw path.
• Spectrogram rendering reuses cached ImageData and lookup buffers.
• Hidden / idle draw and fetch cadences are already lower than the foreground path.

This issue should now focus on the measured remaining paint cost, not the older pre-hardening implementation.

Goal

Profile the current DebugPanel path and trim the remaining spectrogram / waveform paint overhead that still shows up with the panel open.

Actions

• Measure the current panel-open delta against panel-closed behavior.
• Skip spectrogram uploads when nothing new needs to be painted.
• Only redraw waveform / timeline layers when their inputs changed enough to matter.
• If profiling still justifies it, evaluate lower-resolution or off-main-thread rendering for the spectrogram.

Acceptance

• With DebugPanel open, the remaining paint cost is measurably lower than today.
• No new long-task or frame-jank regressions are introduced while reducing panel work.
• Any further optimization is driven by current profiling, not the older trace assumptions.

[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.