Add pluggable metrics architecture with LUFS

Introduce a Metric ABC (compute on worker thread, render on GUI thread)
with a METRICS registry, and refactor the analysis pipeline around it.
RMS power (ported), raw waveform, and BS.1770 LUFS (via pyloudnorm) ship
as the initial three; new metrics drop in by appending to METRICS.

- metrics.py: Metric ABC + RMSPowerMetric, WaveformMetric (locked to
  +/-1.1 y-range for float headroom), LUFSMetric (short-term 3 s window
  + integrated value, with streaming-target reference line).
- plot_control_widget.py: metric selector dropdown + Refresh Plot button
  (moved out of FontControlWidget).
- analysis_results_manager.py: AnalysisResult caches the AudioFile and a
  per-metric data dict; new MetricComputeWorker runs metric switches off
  the GUI thread via metricComputeStarted/metricReady/metricComputeError
  signals, so LUFS on a 12-minute track no longer stalls the UI.
- main.py: all redraw paths funnel through one _render_or_request helper;
  stale-result guards keep slow computes from overwriting fresh selections.
- plotting_engine.py removed (metadata text moved onto AnalysisResult;
  figure construction lives in each Metric).

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

pyproject.toml

@@ -11,6 +11,7 @@ dependencies = [
     "numpy",
     "matplotlib",
     "mutagen",
+    "pyloudnorm",
     "PyQt5>=5.15.10",
     # 5.15.2 is the only pyqt5-qt5 release with a Windows wheel; later
     # versions are Linux/macOS only.
@@ -26,9 +27,10 @@ py-modules = [
     "analysis_results_manager",
     "audio_visualization_widget",
     "master_core",
-    "plotting_engine",
+    "metrics",
     "font_manager",
     "font_control_widget",
+    "plot_control_widget",
     "logger_setup",
     "setup_fonts",
 ]