AetherEngine 2.1.2

Playback fix. Head-of-stream A/V sync. No public API change. Existing 2.1.x callers are unaffected.

Fixed

Audio no longer leads video at the start of a file. On a fresh play (baseIndex 0) the producer snapped the first audio packet onto the video's tfdt (desired 0), which subtracted the audio track's intrinsic start offset from every audio packet. On sources whose first full audio frame lands well past video frame 0 (Cars: EAC3 first frame at +256 ms) this pulled the whole audio track that far ahead of the picture for the entire session, reported as a 256 ms A/V offset in the stats overlay.

Head-of-stream now derives the audio shift from the video's origin shift, so both streams undergo one shared transform and their true source-time relationship is preserved by construction. The audio fragment's tfdt then starts a little after the video's, which fmp4 / AVPlayer represent natively (audio is simply silent for the leading gap). Resume and scrub sessions were unaffected and keep the existing gate-on-video snap (sub-frame residual, part of the HEVC-resume alignment stack).

Verification

Verified on device with Cars: the diagnostic A/V gap now reports 0 ms at head-of-stream (was 256 ms). Build green.

Full changelog: 2.1.1...2.1.2