Function discover_decode_capabilities 

pub fn discover_decode_capabilities() -> DecodeCapabilities

Probe the media backend for hardware decode capabilities.

When the gst-backend feature is enabled, this queries the GStreamer element registry for video decoder elements whose metadata hints at hardware acceleration. The classification uses the element’s klass string ("Hardware" keyword) and a built-in list of known hardware decoder name prefixes — see is_hardware_video_decoder for details.

When the gst-backend feature is disabled, this returns a capabilities struct with hardware_decode_available = false and an empty decoder list.

This function is intentionally cheap — it reads only the plugin registry (no pipeline construction or device probing).