feat(65): F10 saves a clip; the gear drawer owns the three settings

Slice 4. 607 green, 0 warnings. Clips remain **off by default** and, while off, the ring is never
constructed — so no reader thread and no encoder output exist until someone asks for the feature.

- **F10** (`HotkeyId.SaveClip`, default F10, remappable in the hotkey dialog) snapshots the ring, writes
  the file, prunes to the retention count and toasts the name. Empty ring → "nothing to clip yet"
  instead of a zero-byte file; clips off → says so and points at Settings.
- **Settings**: Enable, length (10 or 15, default 10), keep (3/4/5, default 3). Persisted in the layout
  store and **coerced on load** — a bad stored value falls back to a legal one, so the ring can never be
  sized for a length the UI cannot show.
- The ring is sized from length × tier bitrate × 1.25 and rebuilt if either changes; it is created at
  session start and dropped at stop, so memory is only spent while a show is running.
- The drawer states the boundary in the creator's terms: "A capture, not an editor: for trimming,
  captions or a library, use a video editor."

Still to come: the replay layer itself (play once then hide in Live, loop the kept clips elsewhere).

ViewModels/MainViewModel.Scenes.cs

@@ -216,6 +216,7 @@ public partial class MainViewModel
             case Services.HotkeyId.SceneBrb: TransitionToCanonical(SceneCatalog.Brb); break;
             case Services.HotkeyId.SceneChat: TransitionToCanonical(SceneCatalog.Chat); break;
             case Services.HotkeyId.SceneEnding: TransitionToCanonical(SceneCatalog.Ending); break;
+            case Services.HotkeyId.SaveClip: SaveReplayClip(); break;
             case Services.HotkeyId.StartStopStream:
                 if (IsLive || IsRecording)
                 {