feat(topbar): the output stage — Output [YouTube] [Local MP4] One output at a time

Unit B (creator ruling 2026-11-12). Pressing Record / Go Live now changes the bar: the REC/ON-AIR pill
pair becomes two labelled switches with glyphs (the YouTube mark, the record dot), under an "Output"
label and beside "One output at a time". Build world shows nothing but its own segment.

Two things this exposed that were not cosmetic:

- **Local MP4 was not actually the default.** Both pills started unarmed, so the stage opened with no
  output selected; the old screen got away with it because the primary button inferred "record" from
  "nothing armed". RecordPillOn now starts true, which is what "pos = ON (default)" means in code.
- **The stage gates the whole idle cluster, not just the switch.** Gating only the switch left the start
  button visible in the Build world — a Start with no output chosen. ShowOutputStage (output world, idle,
  not recording) wraps the cluster, so a running session hides the stage too: its output cannot change.

Exclusivity stays in the view model (arming one drops the other); the XAML merely reflects it.
TopBarWorldTests pins the stage's visibility and the radio pair starting at Local MP4.

680 pass, 0 warnings. Next: unit C (output-dependent primary action, End labels).
Scope justification: Assets/starting-background.jpg is outside this unit — the creator's replacement art,
still awaiting their visual check.

ytLive.Tests/UiShellTests.cs

@@ -41,6 +41,14 @@ public class UiShellTests
         Assert.Contains("Data=\"M7,10V7a5,5 0 0,1 10,0v3h1", topBar);
         Assert.Contains("Visibility=\"{Binding IsOutputWorld, Converter={StaticResource BoolToVis}}\"", topBar);
 
+        // 2c. The output stage: two labelled switches and the exclusivity promise beside them.
+        Assert.Contains("Text=\"Output\"", topBar);
+        Assert.Contains("Text=\"YouTube\"", topBar);
+        Assert.Contains("Text=\"Local MP4\"", topBar);
+        Assert.Contains("Text=\"One output at a time\"", topBar);
+        Assert.Contains("IsChecked=\"{Binding OnAirPillOn}\"", topBar);
+        Assert.Contains("IsChecked=\"{Binding RecordPillOn}\"", topBar);
+
         // 3. Active scene = orange; live stays red.
         Assert.Contains("IsStaged}\" Value=\"True\">\n                                        <Setter Property=\"BorderBrush\" Value=\"{StaticResource Brush.Attention}\"/>", strip);