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.

Services/LayoutStore.Load.cs

@@ -117,7 +117,7 @@ public partial class LayoutStore : IDisposable
                         TextContent, TextFontFamily, TextFontSize, TextColor, TextBold, TextItalic,
                         TextUnderline, TextAlignment, TextVerticalAlignment, MediaMuted,
                        ChromaKeyEnabled, ChromaKeyColor, ChromaKeySimilarity, ChromaKeySoftness,
-                       ChromaKeySpill, ChromaScreen
+                       ChromaKeySpill, ChromaScreen, IsReplayLayer
                 FROM Source ORDER BY SortOrder
                 """;
             using var reader = cmd.ExecuteReader();
@@ -198,6 +198,8 @@ public partial class LayoutStore : IDisposable
                     ChromaScreen = reader.IsDBNull(reader.GetOrdinal("ChromaScreen"))
                         ? ChromaScreen.Green
                         : (ChromaScreen)reader.GetInt32(reader.GetOrdinal("ChromaScreen")),
+                    IsReplayLayer = !reader.IsDBNull(reader.GetOrdinal("IsReplayLayer"))
+                                    && reader.GetInt32(reader.GetOrdinal("IsReplayLayer")) != 0,
                 };
                 if (!sourcesByScene.TryGetValue(sceneId, out var list))
                     sourcesByScene[sceneId] = list = new List<(Source, int)>();