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.

ViewModels/MainViewModel.Streaming.cs

@@ -41,7 +41,9 @@ public partial class MainViewModel : ViewModelBase
     // ─── IsRecording / IsLive are reality. ON-AIR needs a sign-in, and the
     // ─── pills are radio-exclusive (record-OR-live, creator ruling 2026-09-01):
     // ─── arming one clears the other.
-    private bool _recordPillOn;
+    // Local MP4 is the armed output at launch (creator ruling 2026-11-12: "pos = ON (default)").
+    // Before that both were unarmed, so the output stage opened with nothing selected.
+    private bool _recordPillOn = true;
     private bool _onAirPillOn;
     private bool _isRecording;