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.

Controls/TopBar.xaml

@@ -100,15 +100,10 @@
                 <!-- Go face names the world: "Start Recording" vs "Go Live".
                      (Signed-out ON-AIR Start routes to the sign-in — StartSession.) -->
                 <Button Content="{Binding PrimaryStartButtonLabel}" Style="{StaticResource YtButton}"
-                        Height="32" Margin="10,0,0,0" Command="{Binding StartStreamCommand}"
+                        Height="32" Margin="10,0,0,0" Command="{Binding PrimaryStartCommand}"
                         Visibility="{Binding ShowPrimaryStartButton, Converter={StaticResource BoolToVis}}"/>
-                <!-- Test is a child of Stream — a dry-run of the livestream, so it
-                     exists only inside the Stream world (stream armed + signed in). -->
-                <Button Content="Test" Style="{StaticResource YtButtonSecondary}" Padding="12,4" FontSize="11"
-                        Height="32" Margin="8,0,0,0" VerticalAlignment="Center"
-                        Command="{Binding StartTestCommand}"
-                        ToolTip="Run a private test broadcast to check your chat overlay."
-                        Visibility="{Binding ShowTestButton, Converter={StaticResource BoolToVis}}"/>
+                <!-- No separate TEST button (creator ruling 2026-10-30): EDIT decides whether
+                     the ON-AIR start is the real livestream or the test one. -->
             </StackPanel>
 
             <!-- RUNNING: one reality line (dot + word + elapsed) and the stop -->