feat(scoreboard): the layer is available to ALL scenes (YT_ACTIVITY_TICKER)

Creator ruling: the Scoreboard layer exists in every scene rather than being added five times.
ScoreboardHealer.Ensure keeps exactly one per scene — healed on load like the Background and Brand
logo — so a scene can never end up with two, and 'off' is the ordinary per-scene eye toggle rather
than a delete.

It is appended as the TOPMOST layer on purpose: a full-width strip draws over the scene, and
inserting it above the background would break the Brand logo's documented invariant (directly above
the background). That choice is also why the layer-reorder test still passes untouched.

Also: a layer-list icon for the type, so the row is not blank.

Guard: ScoreboardHealerTests — one per scene, topmost, exact strip geometry, idempotent, and a
duplicate healed back to one.

ytLive.Tests/ScoreboardHealerTests.cs

@@ -0,0 +1,53 @@
+using System.Collections.Generic;
+using System.Linq;
+using Xunit;
+using ytLive.Models;
+
+namespace ytLive.Tests;
+
+/// <summary>Creator ruling (2026-10-30): the Scoreboard layer — YT_ACTIVITY_TICKER — is AVAILABLE TO
+/// ALL SCENES. It therefore heals into every scene like the Background and Brand logo, and a scene
+/// can never end up with two of them.</summary>
+public class ScoreboardHealerTests
+{
+    private static Scene SceneWith(string name)
+    {
+        var scene = new Scene { Name = name };
+        scene.Elements.Add(new Source { Name = "Background", Type = SourceType.Background, IsBackground = true });
+        scene.Elements.Add(new Source { Name = "Brand logo", Type = SourceType.Image, IsBrandLogo = true });
+        return scene;
+    }
+
+    [Fact]
+    public void Every_scene_gets_exactly_one_scoreboard_below_the_background()
+    {
+        var scenes = new List<Scene>
+        {
+            SceneWith(SceneCatalog.Starting), SceneWith(SceneCatalog.Live), SceneWith(SceneCatalog.Brb),
+        };
+
+        Assert.True(ScoreboardHealer.Ensure(scenes));
+        foreach (var scene in scenes)
+        {
+            var boards = scene.Elements.OfType<Source>().Where(s => s.IsScoreboard).ToList();
+            Assert.Single(boards);
+            Assert.Equal("Scoreboard", boards[0].Name);
+            Assert.Equal(0, boards[0].X);
+            Assert.Equal(1920, boards[0].Width);
+            Assert.Equal(ScoreboardHealer.StripHeight, boards[0].Height);
+            // Topmost: a full-width strip over the scene, and the Brand logo keeps its documented
+            // place directly above the background.
+            Assert.Equal(scene.Elements.Count - 1, scene.Elements.IndexOf(boards[0]));
+            Assert.Equal("Brand logo", scene.Elements[1].Name);
+        }
+
+        // Idempotent: a second pass changes nothing (no duplicate strips, no churn).
+        Assert.False(ScoreboardHealer.Ensure(scenes));
+        foreach (var scene in scenes) Assert.Single(scene.Elements.OfType<Source>().Where(s => s.IsScoreboard));
+
+        // A scene that somehow has two is healed back to one.
+        scenes[0].Elements.Add(new Source { Name = "Scoreboard2", Type = SourceType.Scoreboard });
+        Assert.True(ScoreboardHealer.Ensure(scenes));
+        Assert.Single(scenes[0].Elements.OfType<Source>().Where(s => s.IsScoreboard));
+    }
+}