fix(render): cap final mux to video duration

packages/engine/src/services/chunkEncoder.ts

@@ -79,6 +79,8 @@ export interface MuxVideoWithAudioOptions extends Partial<
   audioCodec?: "aac";
   /** Preserve a priming edit list known to have been created by AAC re-encoding. */
   preserveAudioPrimingEditList?: boolean;
+  /** Hard cap copied audio to the already-encoded video's exact duration. */
+  durationSeconds?: number;
 }
 
 async function shouldCopyAacSidecar(
@@ -706,6 +708,9 @@ export async function muxVideoWithAudio(
     // output container metadata. `-c:v copy` is retained; no re-encode.
     args.push("-r", fpsToFfmpegArg(fps));
   }
+  if (config?.durationSeconds !== undefined) {
+    args.push("-t", String(config.durationSeconds));
+  }
   args.push("-y", outputPath);
 
   const processTimeout = config?.ffmpegProcessTimeout ?? DEFAULT_CONFIG.ffmpegProcessTimeout;