refactor(host): extract encode/nvenc_core.rs — shared direct-SDK NVENC leaves

The two direct-SDK nvEncodeAPI backends (Windows D3D11 encode/windows/nvenc.rs,
Linux CUDA encode/linux/nvenc_cuda.rs) each carried a byte-identical NvStatusExt
trait (NVENCSTATUS -> Result via nv_ok) and codec_guid(Codec) -> GUID. Hoist
both into a new encode/nvenc_core.rs, the platform-agnostic sibling of the
existing encode/nvenc_status.rs (same cfg gate: any(linux,windows) + nvenc).
Each backend now imports them via super::nvenc_core; call sites (.nv_ok() ×16/20,
the one codec_guid() struct-init) are unchanged.

The per-platform machinery — entry-table load (nvEncodeAPI64.dll/LoadLibrary vs
libnvidia-encode.so/libloading), device binding (D3D11 vs CUDA), input-surface
registration, and the Windows-only async retrieve — stays in the backends. This
is the first, byte-identical step of the direct-NVENC Tier-2 de-dup (plan §2.2);
the larger build_config authoring is a later, carefully-diffed step.

Verified on BOTH platforms: Linux clippy 0/0 (nvenc,vulkan-encode,pyrowave, RTX
5070 Ti) and Windows clippy 0/0 (nvenc,amf-qsv, RTX 4090 / .173).

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

crates/punktfunk-host/src/encode/windows/nvenc.rs

@@ -36,6 +36,7 @@
 // Every `unsafe` block / impl in this file carries a `// SAFETY:` proof; enforce it.
 #![deny(clippy::undocumented_unsafe_blocks)]
 
+use super::nvenc_core::{codec_guid, NvStatusExt};
 use super::nvenc_status;
 use super::{ChromaFormat, Codec, EncodedFrame, Encoder, EncoderCaps};
 use crate::capture::{CapturedFrame, FramePayload, PixelFormat};
@@ -117,21 +118,6 @@ struct EncodeApi {
     invalidate_ref_frames: unsafe extern "C" fn(*mut c_void, u64) -> nv::NVENCSTATUS,
 }
 
-/// Local `NVENCSTATUS` → `Result` (replaces the sdk's `result_without_string`, which lives in the
-/// crate's `safe` module — code this file must not pull in, see [`EncodeApi`]). The raw status's
-/// Debug repr (`NV_ENC_ERR_INVALID_PARAM`, …) is the error payload.
-trait NvStatusExt {
-    fn nv_ok(self) -> std::result::Result<(), nv::NVENCSTATUS>;
-}
-impl NvStatusExt for nv::NVENCSTATUS {
-    fn nv_ok(self) -> std::result::Result<(), nv::NVENCSTATUS> {
-        match self {
-            nv::NVENCSTATUS::NV_ENC_SUCCESS => Ok(()),
-            err => Err(err),
-        }
-    }
-}
-
 /// Resolve the table once per process. `Err` = NVENC genuinely unavailable on this machine (no
 /// NVIDIA driver/DLL, or a driver older than our headers) — the entry points
 /// ([`NvencD3d11Encoder::open`], [`probe_can_encode_444`]) gate on it and the AMF/QSV/software
@@ -240,15 +226,6 @@ const POOL: usize = 8;
 /// `numRefL0 = 1` keeps each P-frame single-reference for low latency.
 const RFI_DPB: u32 = 5;
 
-fn codec_guid(codec: Codec) -> nv::GUID {
-    match codec {
-        Codec::H264 => nv::NV_ENC_CODEC_H264_GUID,
-        Codec::H265 => nv::NV_ENC_CODEC_HEVC_GUID,
-        Codec::Av1 => nv::NV_ENC_CODEC_AV1_GUID,
-        Codec::PyroWave => unreachable!("PyroWave never opens the direct-NVENC backend"),
-    }
-}
-
 /// Live NVENC hardware-session units held by THIS host process (a plain session = 1; a forced
 /// split-encode session occupies one session per engine = 2–3) — the Stage-W3 encoder budget
 /// (`design/windows-parallel-virtual-displays.md` §4.5). Kept in ONE place so admitting a parallel