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>

analysis_results_manager.py

@@ -214,22 +214,26 @@ class AnalysisResultsManager(QObject):
     self.metric_workers.pop((file_path, metric_id), None)
     self.metricComputeError.emit(file_path, metric_id, error_message)
 
-  def get_metric_figure(self, file_path: str, metric_id: str):
-    """Render a Figure from cached metric data. Returns None if not cached.
+  def get_metric_data(self, file_path: str, metric_id: str):
+    """Return cached metric data, or None if not computed yet.
 
     Never triggers compute — call `request_metric` first and listen for
-    `metricReady` if you need on-demand computation.
+    `metricReady` if you need on-demand computation. Spec/figure building is the
+    GUI layer's job (it owns the view-state), so this stays render-agnostic.
     """
     result = self.results_cache.get(file_path)
     if result is None:
       return None
-    metric = METRICS.get(metric_id)
-    if metric is None:
+    if metric_id not in METRICS:
       return None
-    data = result.metric_data.get(metric_id)
-    if data is None:
-      return None
-    return metric.render(data, file_path)
+    return result.metric_data.get(metric_id)
+
+  def display_label(self, file_path: str) -> str:
+    """Short human label for a file (song name if known, else basename)."""
+    result = self.results_cache.get(file_path)
+    if result is not None and result.song_name:
+      return result.song_name
+    return os.path.basename(file_path)
 
   def get_metadata_text(self, file_path: str) -> str:
     result = self.results_cache.get(file_path)