fix(403): accurate Classic fallback message + engine-coverage tripwire

Review follow-ups (cold review of PR #457):
- Marathon can reach PlayoutBuilder's default arm via `goto default` when
  its enumerator can't be built; the new warning claimed "Marathon is not
  supported by classic scheduling", which is false. Distinguish
  supported-but-failed-to-build (logs "could not build") from genuinely
  unsupported using PlaybackOrderSupport, which also makes Classic a
  runtime consumer of the matrix.
- PlaybackOrderSupportTests now asserts every SchedulingEngineKind has a
  matrix entry, so a new engine kind fails the test instead of throwing
  KeyNotFoundException at runtime.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

ErsatzTV.Core.Tests/Scheduling/PlaybackOrderSupportTests.cs

@@ -28,6 +28,20 @@ public class PlaybackOrderSupportTests
         }
     }
 
+    // Every SchedulingEngineKind must have a matrix entry, or Matrix[engine] throws KeyNotFoundException at
+    // runtime instead of failing here. This is the engine-axis counterpart to the order tripwire.
+    [Test]
+    public void EveryEngineKind_HasAMatrixEntry()
+    {
+        var classified = PlaybackOrderSupport.Engines.ToHashSet();
+
+        foreach (SchedulingEngineKind engine in Enum.GetValues<SchedulingEngineKind>())
+        {
+            classified.ShouldContain(engine,
+                $"SchedulingEngineKind.{engine} has no PlaybackOrderSupport matrix entry (#403).");
+        }
+    }
+
     // The two sets must partition the enum: no order both supported and unsupported, and together they cover
     // exactly the enum (no stale entry for a removed value, no missing value).
     [Test]

ErsatzTV.Core/Scheduling/PlayoutBuilder.cs

@@ -1459,12 +1459,23 @@ public class PlayoutBuilder : IPlayoutBuilder
                 goto default;
 
             default:
-                // An order the classic engine doesn't handle (#403). Say so instead of silently substituting a
-                // different, arbitrary-looking order; keep airing on the Random fallback so the channel does not
-                // go dark on one misconfigured item.
-                _logger.LogWarning(
-                    "Playback order {PlaybackOrder} is not supported by classic scheduling; falling back to Random",
-                    playbackOrder);
+                // Say so instead of silently substituting a different, arbitrary-looking order; keep airing on
+                // the Random fallback so the channel does not go dark on one misconfigured item (#403). A
+                // supported order can also land here via `goto default` (Marathon that couldn't build its
+                // enumerator) -- distinguish the two so the log doesn't claim a supported order is unsupported.
+                if (PlaybackOrderSupport.IsSupported(SchedulingEngineKind.Classic, playbackOrder))
+                {
+                    _logger.LogWarning(
+                        "Could not build a {PlaybackOrder} enumerator for classic scheduling; falling back to Random",
+                        playbackOrder);
+                }
+                else
+                {
+                    _logger.LogWarning(
+                        "Playback order {PlaybackOrder} is not supported by classic scheduling; falling back to Random",
+                        playbackOrder);
+                }
+
                 return new RandomizedMediaCollectionEnumerator(mediaItems, state);
         }
     }