TASK 5 items 1-3: reusable stream, health banner, docs refresh

- Reusable stream: GetOrCreateReusableStreamAsync, cached via LayoutStore, bound at broadcast insert
- Health monitoring: GetStreamHealthAsync polls liveStreams.status, report-by-exception banner
- Go-live order: ensure stream → create broadcast → then start pump (URL must exist first)
- Full broadcast form scoped (TASK 5 items 6-7) in TASKS.md + ai.md
- TASK 23 trimmed to technical scope, TASK 26 added (marcom/launch kit)
- Confidential files: MONETIZATION.md + MARCOM.md (gitignored, not committed)
- .gitignore updated for confidential business files
- HANDOFF.md updated with session state and next steps

MainWindow.xaml

@@ -74,6 +74,7 @@
 
     <Grid>
         <Grid.RowDefinitions>
+            <RowDefinition Height="Auto"/>
             <RowDefinition Height="Auto"/>
             <RowDefinition Height="*"/>
             <RowDefinition Height="Auto"/>
@@ -136,7 +137,16 @@
         </Border>
 
         <!-- ═══ MIDDLE: Main Content ═══ -->
-        <Grid Grid.Row="1" Margin="8">
+        <!-- YouTube health banner (TASK 5 item 3): report-by-exception — visible
+             ONLY while a warning/error configuration issue exists. Amber for
+             warning, darker red for error (HealthIssueBackground). -->
+        <Border Grid.Row="1" Background="{Binding HealthIssueBackground}"
+                Visibility="{Binding HealthIssueBanner, Converter={StaticResource NotNullToVis}}">
+            <TextBlock Text="{Binding HealthIssueBanner}" Foreground="White" FontSize="13"
+                       FontWeight="SemiBold" Margin="16,8" TextTrimming="CharacterEllipsis"/>
+        </Border>
+
+        <Grid Grid.Row="2" Margin="8">
             <Grid.ColumnDefinitions>
                 <ColumnDefinition Width="220"/>
                 <ColumnDefinition Width="*"/>
@@ -869,7 +879,7 @@
         </Grid>
 
         <!-- ═══ BOTTOM BAR: Stream Health + Audio ═══ -->
-        <Border Grid.Row="2" Background="#0f3460" Padding="16,6">
+        <Border Grid.Row="3" Background="#0f3460" Padding="16,6">
             <Grid>
                 <Grid.RowDefinitions>
                     <RowDefinition Height="Auto"/>