feat(webrtc-sys): enable NVENC hardware encoding on Windows

[sionzee]

// accidentally opened PR

The NVIDIA NVENC encoder code already exists in webrtc-sys (h264/h265 encoder impls, cuda_context with Windows LoadLibrary support, NvCodec SDK) but was only compiled on Linux. This enables it on Windows too.

Requires CUDA Toolkit headers at build time (set CUDA_HOME or use default install path). At runtime, loads nvcuda.dll from the GPU driver.

The VideoEncoderFactory composite pattern automatically tries NVIDIA first, falling back to software encoders if CUDA is unavailable.

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[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Link CUDA/NVCUVID symbols for Windows NVENC build

When cuda.h is found on Windows, this block compiles NvDecoder.cpp/NvEncoder*.cpp and enables USE_NVIDIA_VIDEO_CODEC, but the Windows path never adds equivalent symbol resolution for CUDA/NVDEC (nvcuda/nvcuvid) while the Linux path explicitly wires lazy-load stubs via add_lazy_load_so(..., ["cuda", "nvcuvid"]). In CUDA-enabled Windows builds this can surface as unresolved external symbols for cu*/cuvid* during final linking, so the feature being enabled here may fail to build unless explicit Windows linking or lazy-load shims are added.

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