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/producer/src/services/audioExtractor.ts

@@ -88,7 +88,8 @@ export function parseAudioElements(html: string): AudioElement[] {
  */
 function runFFmpeg(args: string[]): Promise<void> {
   return new Promise((resolve, reject) => {
-    const ffmpeg = spawn(getFfmpegBinary(), args);
+    // See runFfmpeg.ts: keeps a console window off the user's desktop on Windows.
+    const ffmpeg = spawn(getFfmpegBinary(), args, { windowsHide: true });
     trackChildProcess(ffmpeg);
     let stderr = "";
 

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

@@ -333,7 +333,11 @@ let cachedFfmpegVersion: string | null = null;
  */
 export async function readFfmpegVersion(): Promise<string> {
   if (cachedFfmpegVersion !== null) return cachedFfmpegVersion;
-  const { stdout } = await execFile("ffmpeg", ["-version"], { maxBuffer: 1024 * 1024 });
+  const { stdout } = await execFile("ffmpeg", ["-version"], {
+    maxBuffer: 1024 * 1024,
+    // See runFfmpeg.ts: keeps a console window off the user's desktop on Windows.
+    windowsHide: true,
+  });
   const firstLine = stdout.split(/\r?\n/)[0]?.trim() ?? "";
   if (!firstLine) {
     throw new Error("ffmpeg -version returned empty output");