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/Audio/AudioSyncDelay.cs

@@ -10,8 +10,10 @@ namespace ytLive.Services.Audio;
 /// the same audio N ms later out — so it is trivially testable and carries no
 /// state outside its own ring.
 ///
-/// Positive offsets only (0..500 ms). Advancing audio (<see cref="TimeSpan"/>
-/// would need a video-side delay and is out of scope for the audio layer.
+/// Positive offsets only (0..500 ms). Advancing audio (the audio runs BEHIND
+/// the video) is handled upstream in <see cref="AudioMixer"/>, not here: the
+/// mixer eats the first |N| ms of the live stream head (OBS's negative sync —
+/// see <see cref="AudioMixer.StartLive"/>), keeping this line a pure delay.
 /// Changing the delay flushes the line, so a live slider change clicks rather
 /// than smearing. Safe to call from any thread — the mixer owns it.
 /// </summary>