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).

Controls/SettingsDrawer.xaml

@@ -63,6 +63,21 @@
             </Popup>
 
             <!-- ═══ ZOOM ═══ -->
+            <TextBlock Text="Clips" Style="{StaticResource SectionHeader}" Margin="0,10,0,4"/>
+            <CheckBox Content="Save replay clips" Style="{StaticResource PropertyPillCheck}" Margin="12,2,0,4"
+                      IsChecked="{Binding ClipsEnabled, Mode=TwoWay}"
+                      ToolTip="F10 saves the last few seconds of the show. Off by default; nothing runs while it is off."/>
+            <StackPanel Margin="12,0,0,4" IsEnabled="{Binding ClipsEnabled}">
+                <TextBlock Text="Clip length" Foreground="{StaticResource Brush.OnSurfaceVariant}" FontSize="11" Margin="0,0,0,2"/>
+                <ComboBox ItemsSource="{Binding ClipLengthOptions}" SelectedItem="{Binding ClipLengthText, Mode=TwoWay}"
+                          Style="{StaticResource YtComboBox}" Height="28" Margin="0,0,0,6"/>
+                <TextBlock Text="Keep" Foreground="{StaticResource Brush.OnSurfaceVariant}" FontSize="11" Margin="0,0,0,2"/>
+                <ComboBox ItemsSource="{Binding ClipKeepOptions}" SelectedItem="{Binding ClipKeepText, Mode=TwoWay}"
+                          Style="{StaticResource YtComboBox}" Height="28"/>
+                <TextBlock Text="A capture, not an editor: for trimming, captions or a library, use a video editor."
+                           Foreground="{StaticResource Brush.OnSurfaceVariant}" FontSize="10" TextWrapping="Wrap" Margin="0,6,0,0"/>
+            </StackPanel>
+
             <TextBlock Text="Zoom" Style="{StaticResource SectionHeader}" Margin="0,6,0,4"/>
 
             <TextBlock Text="Zoom view shape" Foreground="{StaticResource Brush.OnSurfaceVariant}" FontSize="11" Margin="10,0,0,2"/>