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.

ytLive.Tests/SessionStartDecisionTests.cs

@@ -0,0 +1,32 @@
+using Xunit;
+using ytLive.Models;
+
+namespace ytLive.Tests;
+
+/// <summary>Creator ruling (2026-10-30): the top bar's primary button is context-sensitive and the
+/// separate TEST button is gone.
+///
+/// <para>EDIT off + ON-AIR → the livestream message. EDIT on + ON-AIR → the test-livestream
+/// message. REC always records, because there is no test recording. The matrix is pure, so it is
+/// cheaper to pin it here than to drive a window.</para></summary>
+public class SessionStartDecisionTests
+{
+    [Fact]
+    public void Edit_decides_whether_the_on_air_start_is_live_or_a_test()
+    {
+        // ON-AIR armed: EDIT decides.
+        Assert.Equal(SessionStartKind.Live, SessionStartDecision.For(recordArmed: false, onAirArmed: true, editing: false));
+        Assert.Equal("Go Live", SessionStartDecision.LabelFor(SessionStartKind.Live));
+
+        Assert.Equal(SessionStartKind.Test, SessionStartDecision.For(recordArmed: false, onAirArmed: true, editing: true));
+        Assert.Equal("Test Stream", SessionStartDecision.LabelFor(SessionStartKind.Test));
+
+        // REC armed: always a recording, whatever EDIT says (no test recording exists).
+        Assert.Equal(SessionStartKind.Record, SessionStartDecision.For(true, false, false));
+        Assert.Equal(SessionStartKind.Record, SessionStartDecision.For(true, false, true));
+        Assert.Equal("Start Recording", SessionStartDecision.LabelFor(SessionStartKind.Record));
+
+        // Unarmed start records locally (the no-dead-ends rule).
+        Assert.Equal(SessionStartKind.Record, SessionStartDecision.For(false, false, false));
+    }
+}