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/Cargo.toml

@@ -127,3 +127,16 @@ windows = { version = "0.62", features = [
 openh264 = "0.9"
 # WASAPI loopback audio capture (default render endpoint -> 48 kHz stereo f32 for the Opus path).
 wasapi = "0.23"
+# NVENC hardware encoder (NVENC SDK, D3D11 input). The SDK pins `cudarc` with
+# `cuda-version-from-build-system` (a build-time CUDA-toolkit probe); its `ci-check` feature switches
+# cudarc to `dynamic-loading` (loads nvcuda.dll at runtime — nothing needed at build), which is how
+# the crate builds on docs.rs/CI. We enable it so the GPU-less VM/CI compiles; the DirectX NVENC path
+# never calls CUDA at runtime, so the pinned CUDA bindings version is irrelevant.
+nvidia-video-codec-sdk = { version = "0.4", features = ["ci-check"], optional = true }
+
+[features]
+# NVENC hardware encode (Windows). OFF by default: it pulls the NVENC SDK, and the host then needs
+# the NVENC entry points (NvEncodeAPICreateInstance / NvEncodeAPIGetMaxSupportedVersion) at link
+# time — i.e. `nvencodeapi.lib` from the NVIDIA Video Codec SDK (or an import lib generated from
+# nvEncodeAPI64.dll) on the linker path. Build the GPU host with `--features nvenc`.
+nvenc = ["dep:nvidia-video-codec-sdk"]