TRAX: error display, retain track on failure, gray idle dot

1. Error text shows in red inside the desktop audio meter bar (scrollable)
   when TRAX load or playback fails — user gets feedback, not just a
   silent red dot.
2. LoadTrax no longer nulls _music on error — the track path is retained
   so the user sees what failed and can retry.
3. OnTraxPlaybackEnded now receives the NAudio exception and surfaces it.
4. TRAX dot is green (playing) or gray (idle/no track) — no more yellow
   or red states.

207 tests, 0 warnings.

MainWindow.xaml

@@ -924,6 +924,12 @@
                                                    VerticalAlignment="Stretch"/>
                                         <Rectangle Width="2" Fill="#00000080" HorizontalAlignment="Left" Margin="230,0,0,0"
                                                    VerticalAlignment="Stretch"/>
+                                        <ScrollViewer HorizontalScrollBarVisibility="Auto" VerticalScrollBarVisibility="Disabled"
+                                                      Visibility="{Binding TraxError, Converter={StaticResource NotNullToVis}}"
+                                                      VerticalAlignment="Center" Margin="4,0">
+                                            <TextBlock Text="{Binding TraxError}" Foreground="#e94560" FontSize="9"
+                                                       FontWeight="SemiBold" VerticalAlignment="Center"/>
+                                        </ScrollViewer>
                                     </Grid>
                                 </Border>
                                 <Grid Width="16" Height="16" Cursor="Hand" Margin="0,0,8,0"