fix(render): preserve normalized M4A edit timing

packages/engine/src/services/chunkEncoder.test.ts

@@ -418,6 +418,30 @@ describe("muxVideoWithAudio audio codec handling", () => {
     });
   });
 
+  it("preserves M4A priming edit lists instead of shifting copied video", async () => {
+    const { spawn, calls } = createSpawnSpy();
+    vi.resetModules();
+    vi.doMock("child_process", () => ({ spawn }));
+
+    const { muxVideoWithAudio } = await import("./chunkEncoder.js");
+    const muxPromise = muxVideoWithAudio(
+      "/tmp/video-only.mp4",
+      "/tmp/audio.duration-normalized.m4a",
+      "/tmp/output.mp4",
+      undefined,
+      { audioCodec: "aac" },
+      { num: 30, den: 1 },
+    );
+
+    await flushMuxCodecResolution();
+    expect(calls).toHaveLength(1);
+    expect(calls[0]!.args).toContain("copy");
+    expect(calls[0]!.args).not.toContain("-avoid_negative_ts");
+
+    emitClose(calls[0]!.proc, 0);
+    await expect(muxPromise).resolves.toMatchObject({ success: true });
+  });
+
   it("uses the caller-provided AAC codec contract instead of the sidecar extension", async () => {
     const { spawn, calls } = createSpawnSpy();
     vi.resetModules();

packages/engine/src/services/chunkEncoder.ts

@@ -690,9 +690,14 @@ export async function muxVideoWithAudio(
       args.push("-c:a", "aac", "-b:a", "192k", "-movflags", "+faststart");
     }
   }
+  const copiesContainerizedAac =
+    !isWebm && shouldCopyAudio && extname(audioPath).toLowerCase() === ".m4a";
   // PTS bases can diverge during mux and reintroduce negative DTS. See
   // buildEncoderArgs for the full reasoning on why that breaks playback.
-  args.push("-avoid_negative_ts", "make_zero");
+  // A freshly encoded M4A is the exception: its edit list already hides the
+  // AAC priming packet. `make_zero` discards that edit and shifts copied video
+  // forward by one AAC frame (~21ms), creating a visible first-frame offset.
+  if (!copiesContainerizedAac) args.push("-avoid_negative_ts", "make_zero");
   if (fps !== undefined) {
     // Set the exact output framerate so the muxer doesn't PTS-average a
     // fractional rational like `360000/12001` instead of `30/1` into the