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).

ViewModels/MainViewModel.Audio.cs

@@ -155,16 +155,17 @@ public partial class MainViewModel
         private set => SetProperty(ref _micSourceName, value);
     }
 
-    /// <summary>Global audio-sync delay in milliseconds (0..500, default 0),
-    /// applied to the whole mix so audio lands on the video when it runs ahead
-    /// — the OBS fix for lip-sync. Consumed live by the mixer's delay line via
-    /// a Func seam; persisted with the layout.</summary>
+    /// <summary>Global audio-sync offset in milliseconds (−500..500, default 0):
+    /// POSITIVE delays the whole mix so audio lands on the video when it runs
+    /// AHEAD (OBS's lip-sync fix); NEGATIVE advances it by dropping the first
+    /// |N| ms of the live stream head when audio runs BEHIND the video.
+    /// Consumed live by the mixer via a Func seam; persisted with the layout.</summary>
     public int AudioSyncOffsetMs
     {
         get => _audioSyncOffsetMs;
         set
         {
-            var clamped = Math.Clamp(value, 0, 500);
+            var clamped = Math.Clamp(value, -500, 500);
             if (SetProperty(ref _audioSyncOffsetMs, clamped))
                 ScheduleSave();
         }