feat(topbar): the output stage's three states, and keyboard entry into the bar

Build 729 (creator):

- "press <esc> to return" loses the word "press"; the label is "Record / Go-Live".
- The bar takes focus on "Build scenes" and Tab cycles between the two halves only
  (KeyboardNavigation.TabNavigation="Cycle"), never off into the window.
- Entering Record / Go-Live starts with NOTHING chosen — the output switches are a fresh decision,
  not a remembered one.
- Three states: nothing chosen [YouTube][Local MP4]; YouTube chosen [YouTube(red)][Live Stream][Private
  Stream]; Local MP4 chosen [Local MP4(red)][Start Recording] → [End Recording] while recording. Only the
  chosen output is red; the action chips never highlight. Choosing Local MP4 takes the left slot and the
  YouTube chip disappears with it, which is what "slides to the left, replacing it" describes.

"Private Stream" is the private-visibility stream — the same thing this app has always called the test
stream (creator's clarification, so the wording is deliberate, not a new broadcast type).

Output chips are one style with a Tag-driven trigger (OutputChip + OutputChipIcon), so a chip's red face
and its glyph colour can't drift apart; the one-alpha hit-area colour is a token, because
DesignTokensTests rightly refuses literals in theme files.

680 pass, 0 warnings. Scope justification: Assets/starting-background.jpg is outside this unit — the
creator's replacement art, still awaiting their visual check.

ViewModels/MainViewModel.Streaming.cs

@@ -117,6 +117,7 @@ public partial class MainViewModel : ViewModelBase
         {
             if (!SetProperty(ref _recordPillOn, value)) return;
             if (value && _onAirPillOn) OnAirPillOn = false; // record-OR-live — arming REC drops ON-AIR
+            RaiseOutputStage();
             OnPropertyChanged(nameof(ShowPrimaryStartButton));
             OnPropertyChanged(nameof(PrimaryStartKind));
             OnPropertyChanged(nameof(PrimaryStartButtonLabel));
@@ -136,6 +137,7 @@ public partial class MainViewModel : ViewModelBase
             // intent, not reality; the Start face turns this into a sign-in.
             if (!SetProperty(ref _onAirPillOn, value)) return;
             if (value && _recordPillOn) RecordPillOn = false; // record-OR-live — arming ON-AIR drops REC
+            RaiseOutputStage();
             OnPropertyChanged(nameof(ShowPrimaryStartButton));
             OnPropertyChanged(nameof(PrimaryStartKind));
             OnPropertyChanged(nameof(PrimaryStartButtonLabel));