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.

Models/SessionStartDecision.cs

@@ -0,0 +1,38 @@
+namespace ytLive.Models;
+
+/// <summary>What the top bar's primary action button will do.</summary>
+public enum SessionStartKind
+{
+    Record,
+    Live,
+    Test,
+}
+
+/// <summary>Creator ruling (2026-10-30): the primary button is context-sensitive, so the separate
+/// TEST button is gone.
+///
+/// <list type="bullet">
+/// <item>EDIT off + ON-AIR armed → Go Live (the livestream message)</item>
+/// <item>EDIT on + ON-AIR armed → Test Stream (the test-livestream message) — a dry run you
+/// cannot leak, because every broadcast is private pre-v1 anyway</item>
+/// <item>REC armed → Start Recording, whatever EDIT says (there is no test recording)</item>
+/// </list>
+///
+/// <para>Pure so the whole matrix is testable without a window.</para></summary>
+public static class SessionStartDecision
+{
+    public static SessionStartKind For(bool recordArmed, bool onAirArmed, bool editing)
+    {
+        // Neither pill armed still starts something: local recording needs no account, and the
+        // no-dead-ends rule forbids an inert primary button.
+        if (recordArmed || !onAirArmed) return SessionStartKind.Record;
+        return editing ? SessionStartKind.Test : SessionStartKind.Live;
+    }
+
+    public static string LabelFor(SessionStartKind kind) => kind switch
+    {
+        SessionStartKind.Record => "Start Recording",
+        SessionStartKind.Test => "Test Stream",
+        _ => "Go Live",
+    };
+}