test(563): pin per-item guide-group advancement, which mutant b showed was unmeasured

Removing SchedulingEngine.AddCountInternal's _state.AdvanceGuideGroup() left both
guide-group assertions green: the locked-group test only compared inside/outside
the group, and the snapshot only compared item 2 to item 0. Assert the actual
sequence instead.

Refs #563

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_015QqCpYFsKgnAnx6jVwrKiV

ErsatzTV.Core.Tests/Scheduling/Engine/SchedulingEngineTests.cs

@@ -87,6 +87,9 @@ public class SchedulingEngineTests
             item.PlayoutId.ShouldBe(1);
         }
 
+        // outside an EPG group every item opens its own guide group
+        items.Select(i => i.GuideGroup).ShouldBe([1, 2, 3, 4]);
+
         // 30 + 45 + 60 + 20 minutes of content
         TimeSpan scheduled = TimeSpan.FromMinutes(155);
         items[^1].Finish.ShouldBe(Start.UtcDateTime + scheduled);
@@ -350,7 +353,8 @@ public class SchedulingEngineTests
         grouped.Select(i => i.GuideGroup).Distinct().Count().ShouldBe(1);
         grouped.ShouldAllBe(i => i.CustomTitle == "Block");
 
-        items[3].GuideGroup.ShouldNotBe(grouped[0].GuideGroup);
+        // unlocking resumes per-item advancement from the group's number
+        items[3].GuideGroup.ShouldBe(grouped[0].GuideGroup + 1);
         items[3].CustomTitle.ShouldBeNull();
     }
 

ErsatzTV.Tests/Controllers/ScriptedScheduleControllerTests.cs

@@ -106,6 +106,9 @@ public class ScriptedScheduleControllerTests
         items[1].CustomTitle.ShouldBe("Morning Block");
         items[2].CustomTitle.ShouldBeNull();
 
+        // after stop_epg_group every item opens its own guide group again
+        items.Skip(2).Select(i => i.GuideGroup).ShouldBe(Enumerable.Range(2, 11));
+
         // pad_until_exact lands on the requested instant, whatever the machine offset is
         items[6].Finish.ShouldBe(new DateTime(2026, 1, 15, 10, 0, 0, DateTimeKind.Utc));
     }