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>

font_control_widget.py

@@ -5,8 +5,8 @@ Self-contained widget for easy layout management.
 
 import logging
 from typing import List, Dict, Optional
-from PyQt5.QtWidgets import (QWidget, QComboBox, QVBoxLayout, QHBoxLayout, 
-                            QLabel, QSlider, QPushButton, QGroupBox)
+from PyQt5.QtWidgets import (QWidget, QComboBox, QVBoxLayout, QHBoxLayout,
+                            QLabel, QSlider, QGroupBox)
 from PyQt5.QtCore import pyqtSignal, Qt
 
 from font_manager import get_font_manager
@@ -19,13 +19,11 @@ class FontControlWidget(QWidget):
     Provides clustered interface for:
     - Font selection (unified for Qt and matplotlib)
     - Qt font size adjustment
-    - Plot regeneration controls
     """
-    
+
     # Signals
     fontChanged = pyqtSignal(str, str)  # (font_name, font_type)
     fontSizeChanged = pyqtSignal(int)   # font_size
-    plotRefreshRequested = pyqtSignal() # manual refresh request
     
     def __init__(self, parent=None):
         """Initialize the font control widget."""
@@ -59,11 +57,7 @@ class FontControlWidget(QWidget):
         # Font size section
         size_section = self._create_font_size_section()
         group_layout.addWidget(size_section)
-        
-        # Control buttons section
-        button_section = self._create_button_section()
-        group_layout.addWidget(button_section)
-        
+
         layout.addWidget(group_box)
         
     def _create_font_selector_section(self) -> QWidget:
@@ -119,20 +113,6 @@ class FontControlWidget(QWidget):
         
         return section
     
-    def _create_button_section(self) -> QWidget:
-        """Create the control buttons section."""
-        section = QWidget()
-        layout = QHBoxLayout(section)
-        layout.setContentsMargins(0, 0, 0, 0)
-        
-        # Refresh plot button
-        self.refresh_button = QPushButton("Refresh Plot")
-        self.refresh_button.setToolTip("Regenerate current plot with new font settings")
-        self.refresh_button.clicked.connect(self.on_refresh_plot_clicked)
-        layout.addWidget(self.refresh_button)
-        
-        return section
-    
     def refresh_font_list(self):
         """Refresh the list of available fonts."""
         self.logger.debug("Refreshing font list...")
@@ -243,11 +223,6 @@ class FontControlWidget(QWidget):
         # Emit signal for external listeners
         self.fontSizeChanged.emit(size)
     
-    def on_refresh_plot_clicked(self):
-        """Handle manual plot refresh button click."""
-        self.logger.info("Manual plot refresh requested")
-        self.plotRefreshRequested.emit()
-    
     def _apply_font_change(self, font_name: str, font_type: str):
         """Apply the font change to both Qt and matplotlib."""
         try: