Move plotting to pyqtgraph: interactive, overlay-capable render layer

Replace the fire-and-forget matplotlib pipeline (render() -> throwaway Figure ->
canvas teardown) with a three-stage architecture that supports zoom/pan, lin/log
toggling, and multi-file overlay:

  compute(audio_file) -> data        # heavy, worker thread, backend-neutral
  build_spec(data, view) -> PlotSpec # cheap, GUI thread, view-aware
  show_specs([(label, spec, color)]) # pyqtgraph, persistent PlotItem, overlay

- plotspec.py: backend-agnostic descriptors (Curve, Band, HLine, Heatmap,
  AxisSpec, PlotSpec) + ViewState (recompute-free lin/log)
- audio_visualization_widget.py: persistent pyqtgraph plot, never torn down;
  per-dataset colours for overlay; spectrogram log-freq via row resample
  (ImageItem is affine-only); ColorBarItem at a fixed cell
- Compare/overlay driven by file-list checkboxes; stable per-song colour by row
- Custom draggable reference lines (add/clear), persist across redraws
- Axis-constrained scroll zoom: Ctrl=time, Shift=value (_AxisZoomViewBox)
- RMS render no longer per-segment fill_between (was the slow path)

Fixes found in review/testing:
- FillBetweenItem needs penned child curves or it fills nothing (RMS/Waveform
  were blank); band fill verified by pixel count
- band overlay alpha was a no-op (QBrush.color() returns a copy)
- colorbar could stack across renders; now added/removed at a fixed layout cell

Deferred (per scope): stereo retention, deep perf rewrites (eager beat_track,
true-peak/crest loops, shared LUFS), per-song colour picker UI.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

pyproject.toml

@@ -16,6 +16,7 @@ dependencies = [
     # 5.15.2 is the only pyqt5-qt5 release with a Windows wheel; later
     # versions are Linux/macOS only.
     "PyQt5-Qt5==5.15.2 ; sys_platform == 'win32'",
+    "pyqtgraph>=0.14.0",
 ]
 
 [project.scripts]
@@ -28,6 +29,7 @@ py-modules = [
     "audio_visualization_widget",
     "master_core",
     "metrics",
+    "plotspec",
     "font_manager",
     "font_control_widget",
     "plot_control_widget",