fix(encode): classify libav-NVENC open failures by errno, not English strerror text

The bitrate-probe ladder stepped down on format!("{e:#}").contains(
"Invalid argument") — an English substring over the WHOLE context chain,
which also fired on any other wrapped EINVAL (e.g. a CUDA-context errno)
and gated a ~10-step ladder on strerror wording. The root ffmpeg::Error
survives the anyhow chain; downcast and match Error::Other{errno:EINVAL}
instead. Same fix for the intra-refresh ENOSYS probe in the open path.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

crates/pf-encode/src/enc/linux/mod.rs

@@ -468,7 +468,15 @@ impl NvencEncoder {
             // sessions) and reopen this session without intra-refresh; any other failure — and
             // any failure when IR wasn't requested — propagates untouched (the bitrate probe
             // keys on EINVAL, which must not trip the latch).
-            Err(e) if intra_refresh && format!("{e:#}").contains("Function not implemented") => {
+            Err(e)
+                if intra_refresh
+                    && matches!(
+                        e,
+                        ffmpeg::Error::Other {
+                            errno: ffmpeg::util::error::ENOSYS
+                        }
+                    ) =>
+            {
                 tracing::warn!(
                     encoder = name,
                     "NVENC intra-refresh not supported by this GPU — falling back to IDR-only \