feat(stream): one-click Back on air — a new broadcast, honestly labelled (TASK 32·3)

The last resilience slice. When a drop ends the broadcast, YouTube cannot resume it, so the only way
back is a new one — and that is what the app now offers, in one click.

- `Services/BroadcastTitle.cs` — the resume marker is applied to the title the show was using, and any
  existing marker is REPLACED rather than stacked ("Show (resumed)" → "Show (resumed 2)"), with the base
  truncated so the whole title fits YouTube's 100-character limit. Otherwise a channel grows two
  identically named videos and the creator has to explain it.
- `MainViewModel.BackOnAirCommand` — reuses the ordinary go-live path with the saved details and the
  cached stream: new broadcast, new VOD, no dialog. It appears only after a drop ended the session and
  the ordinary start button steps aside, so there is exactly one way forward.
- The running line goes red OFF AIR (not amber) in that state; the button reads "● Back on air".

Tests: `BackOnAirTests` — first re-air marks the title, a second drop counts instead of stacking, the
100-character limit holds, an empty title still works, and the post-drop state exposes exactly one
start affordance. FAST gate: 552 passed, 0 failed, 0 warnings.

TASK 32 is now complete: fragmented MP4, blip retry, poll-driven RECONNECTING, ingest pre-flight, Back
on air.

Controls/TopBar.xaml

@@ -107,6 +107,15 @@
                 <Button Content="{Binding PrimaryStartButtonLabel}" Style="{StaticResource YtButton}"
                         Height="32" Margin="10,0,0,0" Command="{Binding PrimaryStartCommand}"
                         Visibility="{Binding ShowPrimaryStartButton, Converter={StaticResource BoolToVis}}"/>
+                <!-- TASK 32·3: the drop ended this broadcast. YouTube cannot resume a completed one, so
+                     the only honest action is a new broadcast — one click, saved details, and the title
+                     says it is a resume so the channel shows what happened. -->
+                <Button x:Name="BackOnAirButton" Content="{Binding BackOnAirLabel}"
+                        Style="{StaticResource YtButton}" Height="32" Margin="10,0,0,0"
+                        Background="{StaticResource Brush.Primary}"
+                        Command="{Binding BackOnAirCommand}"
+                        ToolTip="Start a new broadcast from your saved details — this begins a new video"
+                        Visibility="{Binding IsOffAirAfterDrop, Converter={StaticResource BoolToVis}}"/>
                 <!-- No separate TEST button (creator ruling 2026-10-30): EDIT decides whether
                      the ON-AIR start is the real livestream or the test one. -->
             </StackPanel>