test(engine): write HDR fixture color tags into the H.264 VUI (#2389)

The SDR-to-HDR extraction tests synthesize their HDR fixture with
-color_trc/-color_primaries flags and rely on the encoder propagating
them into the bitstream. The pinned Windows CI ffmpeg build drops the
transfer on that path, so after #2377 narrowed HDR detection to the
transfer function, the fixture probes as SDR on Windows and both tests
fail (hdrPreflightCount 0). Write the VUI directly with the
h264_metadata bitstream filter so the tag survives on every build.

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

@@ -959,6 +959,12 @@ describe.skipIf(!HAS_FFMPEG)("extractAllVideoFrames on a VFR source", () => {
       "smpte2084",
       "-colorspace",
       "bt2020nc",
+      // The -color_* flags above only tag the container/encoder context;
+      // whether they reach the H.264 VUI depends on the ffmpeg build (the
+      // pinned Windows CI build drops the transfer). Write the VUI directly
+      // so probing reports smpte2084 on every build (9/16/9 = bt2020/PQ/bt2020nc).
+      "-bsf:v",
+      "h264_metadata=colour_primaries=9:transfer_characteristics=16:matrix_coefficients=9",
       src,
     ]);
     if (!synth.success) {