feat(top bar): EDIT decides live vs test on the primary button — the TEST button is gone

Creator ruling (2026-10-30):
- EDIT off + ON-AIR armed → the primary button reads "Go Live" and runs the livestream.
- EDIT on + ON-AIR armed → the same button reads "Test Stream" and runs the test broadcast.
- REC armed (or nothing armed) → "Start Recording", whatever EDIT says; there is no test
  recording, and an unarmed start records locally rather than doing nothing.

So the fifth button was redundant: `ShowTestButton` no longer feeds any control. The decision is a
pure function (`Models/SessionStartDecision`) so the whole matrix is testable without a window;
`PrimaryStartCommand` follows the same kind, so label and action cannot disagree.

Guard: SessionStartDecisionTests pins the four cases, including the no-dead-ends fallback that my
first version got wrong (unarmed returned Live instead of Record — the test caught it).

Note for the v1 gate: the "hide TEST at v1" item is now satisfied by construction — the test path
is a mode of the primary button, not a separate control.

ViewModels/MainViewModel.Scenes.cs

@@ -85,8 +85,12 @@ public partial class MainViewModel
         get => _editingActive;
         set
         {
-            if (SetProperty(ref _editingActive, value))
-                OnPropertyChanged(nameof(IsEditMode));
+            if (!SetProperty(ref _editingActive, value)) return;
+            OnPropertyChanged(nameof(IsEditMode));
+            // EDIT decides whether the ON-AIR start is live or a test (creator ruling 2026-10-30).
+            OnPropertyChanged(nameof(PrimaryStartKind));
+            OnPropertyChanged(nameof(PrimaryStartButtonLabel));
+            OnPropertyChanged(nameof(PrimaryStartCommand));
         }
     }