NVIDIA I420 uploads no longer corrupt chroma

Fixes

• Fix NVIDIA encoder I420 uploads to copy each plane using its actual source stride, avoiding chroma corruption when source frames use padded YUV planes. Also fix the local_video publisher reusing mutable I420 frame storage after handing frames to WebRTC.
• Keep one-frame native video streams as latest-frame queues. Move local video subscriber timing metrics into the example and use the WGPU paint callback as the render boundary.
• Add per-publication video encoder backend selection. Add a video encoder backend availability query. Remove LIVEKIT_PREFERRED_HW_ENCODER in favor of per-publication backend selection.