[tiredIsa]

Adds a C ABI application video decoder (livekit::setExternalVideoDecoder, livekit_ffi_set_external_video_decoder).

• For H.264 the WebRTC decoder factory puts it in front of the internal FFmpeg decoder through CreateVideoDecoderSoftwareFallbackWrapper. Declining a stream (create returns null) or returning LK_EXTERNAL_DECODE_FALLBACK keeps decoding in software; LK_EXTERNAL_DECODE_ERROR requests a keyframe.
• Decoded frames are native WebRTC buffers carrying the application's frame. Video streams opened with external_frames receive them as VideoBufferType::External (FFI EXTERNAL) without CPU conversion or copy; other streams read them back via read_i420.
• The Rust changes are in patches/client-sdk-rust/0001-native-v2-runtime.patch (see its README).

Consumer: syrnike13-app GPU decoding of remote screen shares (Media Foundation / DXVA, any GPU vendor). With three H.264 streams the viewer CPU dropped from 56.8 % to 12–13 % of one core.

Verification: local Windows x64 release build, SDK unit tests 375/375; application native tests 29/29 (Release and ASan) against this SDK.

🤖 Generated with Claude Code

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📒 Files selected for processing (10)

• include/livekit/external_video_decoder.h
• include/livekit/livekit.h
• include/livekit/video_frame.h
• include/livekit/video_stream.h
• patches/client-sdk-rust/0001-native-v2-runtime.patch
• patches/client-sdk-rust/README.md
• src/livekit.cpp
• src/video_frame.cpp
• src/video_stream.cpp
• src/video_utils.cpp

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