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.Streaming.Operations.cs

@@ -164,6 +164,7 @@ public partial class MainViewModel : ViewModelBase
         _liveElapsed = TimeSpan.Zero;
         _recordLength = TimeSpan.Zero;
         StartStepDownWatch();
+        StartClipRing();
 
         var recordStarted = false;
         if (alsoRecord)
@@ -407,6 +408,7 @@ public partial class MainViewModel : ViewModelBase
     {
         CancelStreamRecovery();   // a stop wins over a relaunch in flight
         StopStepDownWatch();
+        StopClipRing();
         RemoveAttentionAlert("stream-recovery");
         var wasLive = IsLive;
         var wasRecording = IsRecording;
@@ -516,6 +518,8 @@ public partial class MainViewModel : ViewModelBase
             BitrateKbps = (int)Math.Round(quality.Bitrate * 1000),
             // Same derivation as the mixer's StartLive call: a dev instance serves its own pipe.
             AudioPipeName = InstanceProfile.AudioPipeName(EncoderOptions.DefaultAudioPipeName),
+            // TASK 65: null when clips are off, which leaves ffmpeg's command line untouched.
+            ClipBuffer = _clipRing,
         };
     }