fix(top bar): a signed-out ON-AIR press starts sign-in instead of disabling the button

Reported: EDIT on + ON-AIR on, signed out → the button reads "Test Stream" and cannot be pressed.
Cause: the primary button bound straight to StartTestCommand, whose CanExecute needs a connection.
StartSession already routed signed-out ON-AIR to the sign-in flow; the test path bypassed it.

The route is now explicit and pure: RouteFor(kind, connected) → SignIn | Record | Live | Test. The
primary command calls SignInAsync for the SignIn route, BeginTestStream for Test, StartSession
otherwise — so the label, the action and the sign-in fallback cannot drift apart. Pinned in
SessionStartDecisionTests for both live and test, connected and not.

Models/SessionStartDecision.cs

@@ -29,6 +29,24 @@ public static class SessionStartDecision
         return editing ? SessionStartKind.Test : SessionStartKind.Live;
     }
 
+    /// <summary>What pressing the button actually does, given whether YouTube is connected.
+    /// A signed-out ON-AIR press must start the SIGN-IN flow, not sit there disabled — a dead
+    /// primary button is the dead end the design principle forbids (creator, 2026-10-30).</summary>
+    public enum Route
+    {
+        SignIn,
+        Record,
+        Live,
+        Test,
+    }
+
+    public static Route RouteFor(SessionStartKind kind, bool connected) => kind switch
+    {
+        SessionStartKind.Record => Route.Record,
+        SessionStartKind.Test => connected ? Route.Test : Route.SignIn,
+        _ => connected ? Route.Live : Route.SignIn,
+    };
+
     public static string LabelFor(SessionStartKind kind) => kind switch
     {
         SessionStartKind.Record => "Start Recording",