fix(engine): stop destroying the AAC priming edit list when muxing (#3505)

`muxVideoWithAudio` passed `-avoid_negative_ts make_zero` unless the caller
set `preserveAudioPrimingEditList`. In practice the dominant path is an AAC
sidecar copied into mp4, where that flag is actively harmful: ffmpeg's
default is `auto`, which the mp4/mov muxers (AVFMT_TS_NEGATIVE) already
resolve to `disabled`. Forcing `make_zero` overrides the correct default,
discards the priming edit list the sidecar encode created, shifts the video
start_time forward by one AAC frame and writes an empty video edit at t=0 —
which edit-list-honoring players (QuickTime/Safari) render as a black first
frame.

Verified with ffprobe on a copy mux of a 30fps h264 mp4 and an AAC sidecar:

  with `make_zero`   video start_time 0.066000, elst: [media time -1,
                     dur 5940] + [media time 6000, dur 180000]
                     audio start_time 0.042993, elst: [media time -1, ...]
  without (this fix) video start_time 0.000000, elst: [media time 6000,
                     dur 180000]
                     audio start_time 0.000000, elst: [media time 1024, ...]

The empty leading edit and the offset both disappear, and the audio keeps
its 1024-sample priming edit.

The flag is now never passed for a mux, in any mode. `preserveAudioPrimingEditList`
is part of the exported engine API, so it stays on `MuxVideoWithAudioOptions`
as `@deprecated` and no-op rather than being removed; the two internal callers
that set it (`assembleStage`, distributed `assemble`) drop it.

`buildEncoderArgs` and `streamingEncoder` still pass the flag for video-only
output and are deliberately left alone — those chunks are consumed as
intermediates, not as a delivered mp4/mov.

Fixes #3487

Co-authored-by: Alexandru Mincu <alex@mountsoftware.ro>
Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

packages/producer/src/services/distributed/assemble.ts

@@ -302,10 +302,7 @@ export async function assemble(
     }
 
     // ── 3. Audio: pad-or-trim then mux ────────────────────────────────────
-    let normalizedAudio: {
-      path: string;
-      preserveAudioPrimingEditList: boolean;
-    } | null = null;
+    let normalizedAudioPath: string | null = null;
     if (audioPath !== null && existsSync(audioPath)) {
       const paddedAudioPath = join(workDir, "audio-padded.m4a");
       const padTrimResult = await padOrTrimAudioToVideoFrameCount({
@@ -317,10 +314,7 @@ export async function assemble(
       if (!padTrimResult.success) {
         throw new Error(`[assemble] audio pad/trim failed: ${padTrimResult.error}`);
       }
-      normalizedAudio = {
-        path: paddedAudioPath,
-        preserveAudioPrimingEditList: padTrimResult.operation !== "copy",
-      };
+      normalizedAudioPath = paddedAudioPath;
       log.info("[assemble] audio normalized for mux", {
         operation: padTrimResult.operation,
         targetDurationSeconds: padTrimResult.targetDurationSeconds,
@@ -333,16 +327,17 @@ export async function assemble(
     // because it operates on a `RenderJob` and emits `updateJobStatus`
     // payloads — the distributed activity has no job to thread through.
     const muxOutputPath =
-      normalizedAudio !== null ? join(workDir, `mux.${plan.dimensions.format}`) : postConcatPath;
-    if (normalizedAudio !== null) {
+      normalizedAudioPath !== null
+        ? join(workDir, `mux.${plan.dimensions.format}`)
+        : postConcatPath;
+    if (normalizedAudioPath !== null) {
       const muxResult = await muxVideoWithAudio(
         postConcatPath,
-        normalizedAudio.path,
+        normalizedAudioPath,
         muxOutputPath,
         abortSignal,
         {
           audioCodec: "aac",
-          preserveAudioPrimingEditList: normalizedAudio.preserveAudioPrimingEditList,
         },
         { num: plan.dimensions.fpsNum, den: plan.dimensions.fpsDen },
       );

packages/producer/src/services/render/stages/assembleStage.test.ts

@@ -69,7 +69,7 @@ describe("runAssembleStage audio duration parity", () => {
       "/tmp/audio.duration-normalized.m4a",
       "/tmp/output.mp4",
       undefined,
-      { audioCodec: "aac", preserveAudioPrimingEditList: true },
+      { audioCodec: "aac" },
       { num: 30, den: 1 },
     );
   });

packages/producer/src/services/render/stages/assembleStage.ts

@@ -78,7 +78,6 @@ export async function runAssembleStage(input: AssembleStageInput): Promise<Assem
       abortSignal,
       {
         audioCodec: "aac",
-        preserveAudioPrimingEditList: normalizeResult.operation !== "copy",
       },
       job.config.fps,
     );