feat: video loader + combiner nodes

Adapted from HunyuanVideo-FoleyTune for the audio-SR workflow:

- UniverSR Load Video Audio: extract a video's audio track via ffmpeg
  (WAV pipe + soundfile, no torchcodec) and carry a UNIVERSR_VIDEO
  reference forward, with an inline video preview.
- UniverSR Video Combiner: mux the enhanced audio back onto the source
  video without re-encoding video (-c:v copy), trim-aware, with output
  auto-increment and preview.

Both registered alongside the SR nodes; ffmpeg + soundfile required only
for these. Adds README docs and an example video workflow.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

requirements.txt

@@ -6,3 +6,6 @@ einops>=0.7
 timm>=0.9
 huggingface_hub>=0.20
 pyyaml>=6.0
+# Video loader/combiner nodes only — read/write WAV for ffmpeg muxing.
+# (ffmpeg itself must be installed separately and on PATH.)
+soundfile>=0.12