fix(producer): audio always waits for video gate, even on initial start

The producer's audio gate used to skip the "wait for video" branch on
initial-start sessions, on the assumption that the video gate opens at
the file's first packet and audio + video naturally land at the same
source-time. That assumption broke after 0842e07 made the video gate
always require AV_PKT_FLAG_KEY: on MKV remuxes whose first decode-order
packet isn't a sync sample (e.g. Bluey BD remuxes), the video gate now
skips leading non-key packets and opens at the first IDR. Audio still
anchored itself at its own first packet (much earlier in source-time),
so after the per-stream PTS shift, audio's tfdt=0 represents the
file's start while video's tfdt=0 represents the IDR location. AVPlayer
plays them as simultaneous at time 0, but the source-content is offset
by `firstKey - firstAudio` seconds — Vincent saw "audio kommt später"
(audio playing the silent pre-action intro while video shows the
post-intro action, then ongoing desync once audio "catches up" to a
section it doesn't match).

Fix: `audioWaitForVideo` is now always true at producer init. The
video gate-open block sets `restartTargetAudioDts` from the actual
video DTS and clears the wait. Audio packets land at the first DTS
at-or-after the video keyframe, so both streams' first kept sample
comes from the same source-time and the shift-to-tfdt-0 alignment
holds.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: superuser404notfound

Sources/AetherEngine/Video/HLSSegmentProducer.swift

@@ -308,7 +308,15 @@ final class HLSSegmentProducer: @unchecked Sendable {
         // first kept audio sample is from the same source-time as
         // the first video keyframe we land on.
         self.restartTargetAudioDts = Int64.min
-        self.audioWaitForVideo = (restartTargetVideoDts != Int64.min)
+        // Audio always waits for video, even on initial-start. The video
+        // gate may skip leading non-key packets while scanning for the
+        // first AV_PKT_FLAG_KEY (some MKV remuxes have a non-IDR first
+        // packet, e.g. Bluey BD remuxes); if audio anchored itself at
+        // its own first packet in the meantime, the two streams' first
+        // kept sample would come from different source-times and play
+        // back desynced by `firstVideoKeyDts - firstAudioDts` even
+        // though their tfdts after shift both equal 0.
+        self.audioWaitForVideo = true
         self.desiredFirstVideoTfdtPts = desiredFirstVideoTfdtPts
         self.desiredFirstAudioTfdtPts = desiredFirstAudioTfdtPts