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>

uv.lock

@@ -272,6 +272,15 @@ wheels = [
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 ]
 
+[[package]]
+name = "colorama"
+version = "0.4.6"
+source = { registry = "https://pypi.org/simple" }
+sdist = { url = "https://files.pythonhosted.org/packages/d8/53/6f443c9a4a8358a93a6792e2acffb9d9d5cb0a5cfd8802644b7b1c9a02e4/colorama-0.4.6.tar.gz", hash = "sha256:08695f5cb7ed6e0531a20572697297273c47b8cae5a63ffc6d6ed5c201be6e44", size = 27697, upload-time = "2022-10-25T02:36:22.414Z" }
+wheels = [
+    { url = "https://files.pythonhosted.org/packages/d1/d6/3965ed04c63042e047cb6a3e6ed1a63a35087b6a609aa3a15ed8ac56c221/colorama-0.4.6-py2.py3-none-any.whl", hash = "sha256:4f1d9991f5acc0ca119f9d443620b77f9d6b33703e51011c16baf57afb285fc6", size = 25335, upload-time = "2022-10-25T02:36:20.889Z" },
+]
+
 [[package]]
 name = "contourpy"
 version = "1.3.2"
@@ -1272,6 +1281,19 @@ wheels = [
     { url = "https://files.pythonhosted.org/packages/7f/21/8486ed45977be615ec5371b24b47298b1cb0e1a455b419eddd0215078dba/pyqt5_sip-12.18.0-cp314-cp314-win_amd64.whl", hash = "sha256:6d948f1be619c645cd3bda54952bfdc1aef7c79242dccea6a6858748e61114b9", size = 59622, upload-time = "2026-01-13T15:53:17.714Z" },
 ]
 
+[[package]]
+name = "pyqtgraph"
+version = "0.14.0"
+source = { registry = "https://pypi.org/simple" }
+dependencies = [
+    { name = "colorama" },
+    { name = "numpy", version = "2.2.6", source = { registry = "https://pypi.org/simple" }, marker = "python_full_version < '3.11'" },
+    { name = "numpy", version = "2.4.6", source = { registry = "https://pypi.org/simple" }, marker = "python_full_version >= '3.11'" },
+]
+wheels = [
+    { url = "https://files.pythonhosted.org/packages/32/36/4c242f81fdcbfa4fb62a5645f6af79191f4097a0577bd5460c24f19cc4ef/pyqtgraph-0.14.0-py3-none-any.whl", hash = "sha256:7abb7c3e17362add64f8711b474dffac5e7b0e9245abdf992e9a44119b7aa4f5", size = 1924755, upload-time = "2025-11-16T19:43:22.251Z" },
+]
+
 [[package]]
 name = "python-dateutil"
 version = "2.9.0.post0"
@@ -1660,6 +1682,7 @@ dependencies = [
     { name = "pyloudnorm" },
     { name = "pyqt5" },
     { name = "pyqt5-qt5", marker = "sys_platform == 'win32'" },
+    { name = "pyqtgraph" },
 ]
 
 [package.metadata]
@@ -1671,6 +1694,7 @@ requires-dist = [
     { name = "pyloudnorm" },
     { name = "pyqt5", specifier = ">=5.15.10" },
     { name = "pyqt5-qt5", marker = "sys_platform == 'win32'", specifier = "==5.15.2" },
+    { name = "pyqtgraph", specifier = ">=0.14.0" },
 ]
 
 [[package]]