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.
