feat(65): the replay layer exists in the model; the exclusion problem is solved by the guard

Slice 5a, scaffolding only. 607 green.

- `Source.IsReplayLayer` + schema column + roundtrip: a media layer that shows saved clips instead of a
  chosen file. The compositor needs to know nothing — it is a media layer whose path the VM owns.
- **Two spec corrections from the creator (2026-11-10), recorded in the task:**
  1. The immediate replay of a fresh clip happens **only in the Live scene**. While it plays, **F10 is
     inert** ("replay in progress").
  2. That guard replaces the "layer excluded from its own clip" idea, which is **impossible** with one
     encoder and two muxes: the clip is a second mux of the same frames, so it necessarily contains what
     viewers saw. No second encode, no fiction about a clean feed.

Remaining for 65: the add-layer menu entry, `ClipSaved` → play (Live: once then hide; elsewhere: loop the
kept set), `SessionCompleted` → advance or hide, and silent-by-default audio.

Models/Source.cs

@@ -234,6 +234,16 @@ public class Source : SceneElement
         set => Set(ref _mediaType, value);
     }
 
+    private bool _isReplayLayer;
+
+    /// <summary>TASK 65: this media layer shows saved replay clips rather than a chosen file. The VM
+    /// owns its path and playlist; the compositor needs to know nothing.</summary>
+    public bool IsReplayLayer
+    {
+        get => _isReplayLayer;
+        set => Set(ref _isReplayLayer, value);
+    }
+
     private string? _mediaPath;
     public string? MediaPath
     {