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).

ytLive.Tests/AudioSyncDelayTests.cs

@@ -6,8 +6,9 @@ namespace ytLive.Tests;
 /// <summary>
 /// TASK 22: the global audio-sync delay line. It is a pure fixed positive
 /// delay on the interleaved stereo mix — the OBS fix for audio running ahead
-/// of video. Positive-only: advancing audio needs a video-side delay and is
-/// out of the audio layer's scope.
+/// of video. Positive-only: the line clamps negatives to 0; the NEGATIVE
+/// (advance) side — audio running behind video, the OBS "eat the buffer head"
+/// case — is covered by <see cref="AudioPipelineTests.StartLive_NegativeOffset_AdvancesAudio_ByDroppingTheStreamHead"/>.
 /// </summary>
 public class AudioSyncDelayTests
 {