feat(65): the replay layer plays — once in Live, looping everywhere else

608 green, 0 warnings. Completes the layer half of TASK 65.

- **(+) → Replay Clip** adds an ordinary media layer with an app-owned path, one per scene, silent by
  default (`MediaMuted`) and looping only outside Live — the compositor needs no special case.
- **F10 saves → the Live layer plays it once, then hides** (`SessionCompleted` → invisible, guard
  cleared). Every other scene with a replay layer keeps looping the kept set, advancing on each clip's
  completion, wrapping at the end.
- **The guard the creator asked for:** while that Live replay is playing, **F10 is inert** with a plain
  explanation. That is what replaces the impossible "exclude the layer from its own clip" — with one
  encoder and two muxes the clip *is* the program, so the fix is to not clip a clip.

`ReplayLayerTests` drives the real VM: one layer per scene, muted by default, once-through in Live,
looping in BRB, and a stray F10 with clips off writes nothing.

ViewModels/MainViewModel.Media.cs

@@ -278,6 +278,7 @@ public partial class MainViewModel
             {
                 source.IsMediaRunning = false;
                 source.MediaPlaybackState = MediaPlaybackState.Stopped;
+                if (source.IsReplayLayer) OnReplayLayerCompleted(source);
             }
     }