fix(render): scope M4A priming preservation to trims

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

@@ -290,6 +290,7 @@ export async function assemble(
 
     // ── 3. Audio: pad-or-trim then mux ────────────────────────────────────
     let audioForMux: string | null = null;
+    let preserveAudioPrimingEditList = false;
     if (audioPath !== null && existsSync(audioPath)) {
       const paddedAudioPath = join(workDir, "audio-padded.m4a");
       const padTrimResult = await padOrTrimAudioToVideoFrameCount({
@@ -302,6 +303,7 @@ export async function assemble(
         throw new Error(`[assemble] audio pad/trim failed: ${padTrimResult.error}`);
       }
       audioForMux = paddedAudioPath;
+      preserveAudioPrimingEditList = padTrimResult.operation === "trim";
       log.info("[assemble] audio normalized for mux", {
         operation: padTrimResult.operation,
         targetDurationSeconds: padTrimResult.targetDurationSeconds,
@@ -321,7 +323,7 @@ export async function assemble(
         audioForMux,
         muxOutputPath,
         abortSignal,
-        { audioCodec: "aac" },
+        { audioCodec: "aac", preserveAudioPrimingEditList },
         { num: plan.dimensions.fpsNum, den: plan.dimensions.fpsDen },
       );
       if (!muxResult.success) {

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" },
+      { audioCodec: "aac", preserveAudioPrimingEditList: true },
       { num: 30, den: 1 },
     );
   });

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

@@ -76,7 +76,10 @@ export async function runAssembleStage(input: AssembleStageInput): Promise<Assem
       normalizeResult.outputPath,
       outputPath,
       abortSignal,
-      { audioCodec: "aac" },
+      {
+        audioCodec: "aac",
+        preserveAudioPrimingEditList: normalizeResult.operation === "trim",
+      },
       job.config.fps,
     );
     assertNotAborted();