feat(host/windows): NVENC D3D11 hardware encoder (--features nvenc)

Zero-copy capture->encode on the GPU via the raw NVENC API (nvidia_video_codec_sdk sys + ENCODE_API; the safe wrapper is CUDA-only). Opens an NV_ENC_DEVICE_TYPE_DIRECTX session on the SAME ID3D11Device as the DXGI capturer (carried on the new FramePayload::D3d11), registers a pool of BGRA textures once, CopyResources each captured texture in and encode_picture; CBR/ULL, infinite GOP, P-only, forced-IDR for RFI. The DXGI capturer gains a D3D11 zero-copy output (selected, like the encoder, by PUNKTFUNK_ENCODER=nvenc) so capture+encode share textures.

OFF by default (the nvenc feature pulls the NVENC SDK + cudarc): the default Windows host links without it (openh264 path). cudarc builds toolkit-less via the SDK ci-check feature (dynamic-loading). At link time --features nvenc needs nvencodeapi.lib (NVENC SDK, or an import lib generated from the driver's nvEncodeAPI64.dll) on PUNKTFUNK_NVENC_LIB_DIR. Both default and --features nvenc builds validated to compile+link GPU-less on the VM (import lib generated from the driver DLL). Runtime needs a real NVIDIA GPU.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

crates/punktfunk-host/src/encode.rs

@@ -162,15 +162,26 @@ pub fn open_video(
             .unwrap_or_default()
             .to_ascii_lowercase();
         if matches!(pref.as_str(), "nvenc" | "hw" | "nvidia") {
-            anyhow::bail!(
-                "NVENC hardware encode is not yet implemented on Windows — omit PUNKTFUNK_ENCODER \
-                 or set it to 'software' to use the openh264 encoder"
-            );
+            // Hardware path: NVENC over D3D11. The DXGI capturer switches to its zero-copy
+            // FramePayload::D3d11 output under the same env var so capture + encode share textures.
+            #[cfg(feature = "nvenc")]
+            {
+                let enc =
+                    nvenc::NvencD3d11Encoder::open(codec, format, width, height, fps, bitrate_bps)?;
+                return Ok(Box::new(enc) as Box<dyn Encoder>);
+            }
+            #[cfg(not(feature = "nvenc"))]
+            {
+                anyhow::bail!(
+                    "NVENC requested but this host was built without it — rebuild with \
+                     `--features nvenc` (needs the NVENC SDK's nvencodeapi.lib at link time)"
+                );
+            }
         }
         anyhow::ensure!(
             codec == Codec::H264,
             "the Windows software encoder supports H.264 only; client negotiated {codec:?} \
-             (request H264, or use a GPU host once NVENC lands)"
+             (set PUNKTFUNK_ENCODER=nvenc for a GPU host, or request H264)"
         );
         // Software H.264 realistically caps far below the negotiated hardware rates.
         const SW_BITRATE_CEIL: u64 = 100_000_000;
@@ -189,6 +200,8 @@ pub fn open_video(
 mod linux;
 #[cfg(target_os = "windows")]
 mod sw;
+#[cfg(all(target_os = "windows", feature = "nvenc"))]
+mod nvenc;
 
 #[cfg(test)]
 mod tests {