feat: signed A/V sync offset (−500..+500), negative advances by eating live stream head (OBS eat-head semantics)

Positive offsets still delay the whole mix via the delay line (lip-sync fix);
negative offsets now ARM once at StartLive and drop |N| ms off the pipe's write
head so audio events land earlier when audio runs BEHIND video. Slider relabeled
AUDIO SYNC, Min −500, locked while live/recording (IsEditMode). LayoutStore and
VM clamp to −500..500.

OBS reference for eat-the-head negative sync: https://obsproject.com/kb/obs-studio/buffering-time (negative sync values pull audio earlier by discarding buffered player audio).

Test: StartLive_NegativeOffset_AdvancesAudio_ByDroppingTheStreamHead (6x0.9 head
must be eaten before 0.2 bed reaches the wire).

Services/LayoutStore.Settings.cs

@@ -29,17 +29,19 @@ public partial class LayoutStore : IDisposable
         insert.ExecuteNonQuery();
     }
 
-    /// <summary>The global audio-sync delay in milliseconds (TASK 22), 0..500,
-    /// default 0. Restored at startup so the mixer starts on the user's offset.</summary>
+    /// <summary>The global audio-sync offset in milliseconds (TASK 22), −500..500,
+    /// default 0: positive delays the mix (audio ahead of video), negative
+    /// advances it (audio behind video). Restored at startup so the mixer starts
+    /// on the user's offset.</summary>
     public int LoadAudioSyncOffsetMs()
     {
         var value = GetSetting("Audio.SyncOffsetMs");
-        return int.TryParse(value, out var ms) ? Math.Clamp(ms, 0, 500) : 0;
+        return int.TryParse(value, out var ms) ? Math.Clamp(ms, -500, 500) : 0;
     }
 
     public void SaveAudioSyncOffsetMs(int ms)
     {
-        UpsertSetting("Audio.SyncOffsetMs", Math.Clamp(ms, 0, 500).ToString());
+        UpsertSetting("Audio.SyncOffsetMs", Math.Clamp(ms, -500, 500).ToString());
     }
 
     /// <summary>The user's chosen record output folder (TASK 18), or null to use the