[Impl] Solve vtdec_hw cap negotiation issue for GStreamer pipeline

[saitoomituru]

Atomic task, small context.

From README + cross-review: GStreamer RTSP→NDI works on software decode (CPU 30-35%). vtdec_hw blocked by capability negotiation.

Task: Investigate and fix (or document workaround) the cap nego problem so hardware decode path can be enabled. Update README with current status.

Parent: #1

Keep the heavy-lift battleship running smooth, cowboy.