Expand description
Stub Intel QSV encoder, compiled when the qsv feature is off.
Keeps qsv::QsvEncoder a real type so the dispatcher in
encode/mod.rs compiles unchanged, but construction always errors —
auto-select then skips the Intel tier, and an explicit
EncoderBackend::Qsv request surfaces the error. Enable --features qsv to compile the real oneVPL-backed encoder (qsv.rs).