test(pf-encode): pin the typed-EINVAL classifier's chain-survival contract (Phase 8)

Rides on 25765c53 (the concurrent session landed the same typed-errno
fix first — this branch's twin commit was dropped in the rebase; these
are its surviving residuals):
- Two tests pinning what the ladder's step-down rests on: the typed
  ffmpeg::Error survives with_context layers as a downcastable source
  (an eager format! anywhere between open_with and the ladder would
  silently break it), and the phrase WITHOUT the type no longer
  classifies.
- The CudaHw::new eager-format guard note: those bail!s must NOT be
  converted to typed errors — a hwdevice/hwframes EINVAL is a config
  error no bitrate can fix, and enrolling it would burn ~10 doomed
  opens before surfacing the real failure.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

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

@@ -66,6 +66,11 @@ struct CudaHw {
 
 impl CudaHw {
     /// Build a CUDA hwdevice wrapping `cu_ctx` and a frames pool (`sw_format` = `pixel`).
+    ///
+    /// The `bail!`s below format raw AVERROR ints eagerly BY DESIGN — do not convert them to
+    /// typed errors: `open_nvenc_probed`'s bitrate ladder steps down on a typed EINVAL
+    /// (`nvenc_open_einval`), and a hwdevice/hwframes EINVAL is a config error no bitrate can
+    /// fix — enrolling it would burn ~10 doomed encoder opens before surfacing the real failure.
     unsafe fn new(cu_ctx: *mut std::ffi::c_void, sw_format: Pixel, w: u32, h: u32) -> Result<Self> {
         let mut device_ref = ffi::av_hwdevice_ctx_alloc(ffi::AVHWDeviceType::AV_HWDEVICE_TYPE_CUDA);
         if device_ref.is_null() {