fix(engine): hide ffmpeg console windows on Windows (#3381)

ffmpeg and ffprobe are console-subsystem binaries and Node defaults
windowsHide to false, so every spawn opened a visible console window on
Windows. A render shells out dozens of times across parallel workers,
which flashed a burst of windows across the user's desktop.

Applied at every production spawn site rather than only the two named in
the report, since they all share the cause: runFfmpeg, both gpuEncoder
probes, ffprobe, streamingEncoder, audioExtractor and the distributed
version check. windowsHide is a no-op on macOS and Linux.

The dev-only parity and regression harnesses are left alone; they never
run on a user's desktop.

Closes #3379

packages/engine/src/utils/runFfmpeg.ts

@@ -92,7 +92,11 @@ export function formatFfmpegError(
 
 export async function runFfmpeg(args: string[], opts?: RunFfmpegOptions): Promise<RunFfmpegResult> {
   const timeout = opts?.timeout ?? DEFAULT_TIMEOUT;
-  const ffmpeg = spawn(getFfmpegBinary(), args);
+  // windowsHide: ffmpeg/ffprobe are console-subsystem binaries, so without
+  // this Node opens a visible console window per spawn on Windows. A render
+  // shells out dozens of times across parallel workers, which flashes a burst
+  // of windows across the user's desktop. No-op on macOS and Linux.
+  const ffmpeg = spawn(getFfmpegBinary(), args, { windowsHide: true });
   trackChildProcess(ffmpeg);
   const managed = new ManagedChildProcess(ffmpeg, {
     signal: opts?.signal,