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).

ytLive.Tests/ClipLibraryTests.cs

@@ -74,3 +74,52 @@ public sealed class ClipLibraryTests : IDisposable
         Assert.True(tenAt8 < 15L * 1024 * 1024);               // and still modest
     }
 }
+
+/// <summary>
+/// TASK 65 slice 4 — the three settings behind the gear drawer. They persist, and the loader coerces
+/// anything unexpected back into the allowed set: a bad value must never leave the user with a clip
+/// length the ring cannot serve.
+/// </summary>
+public sealed class ClipSettingsTests : IDisposable
+{
+    private readonly string _db = Path.Combine(Path.GetTempPath(), $"ytLlive-clipsettings-{Guid.NewGuid():N}.db");
+
+    public void Dispose()
+    {
+        Microsoft.Data.Sqlite.SqliteConnection.ClearAllPools();
+        try { File.Delete(_db); } catch { /* best effort */ }
+    }
+
+    [Fact]
+    public void SettingsPersistAndCoerce()
+    {
+        using (var store = new LayoutStore(_db))
+        {
+            Assert.False(store.LoadClipsEnabled());     // opt-in
+            Assert.Equal(10, store.LoadClipSeconds());  // default 10
+            Assert.Equal(3, store.LoadClipKeep());      // default 3
+
+            store.SaveClipsEnabled(true);
+            store.SaveClipSeconds(15);
+            store.SaveClipKeep(5);
+        }
+
+        Microsoft.Data.Sqlite.SqliteConnection.ClearAllPools();
+        using (var store = new LayoutStore(_db))
+        {
+            Assert.True(store.LoadClipsEnabled());
+            Assert.Equal(15, store.LoadClipSeconds());
+            Assert.Equal(5, store.LoadClipKeep());
+
+            store.SaveClipSeconds(7);                    // not a choice we offer
+            store.SaveClipKeep(9);                       // past the ceiling
+        }
+
+        Microsoft.Data.Sqlite.SqliteConnection.ClearAllPools();
+        using (var store = new LayoutStore(_db))
+        {
+            Assert.Equal(10, store.LoadClipSeconds());
+            Assert.Equal(3, store.LoadClipKeep());
+        }
+    }
+}