gpu_video_encoder's encoder/vaapi/vk_device/dmabuf modules are #[cfg(linux)]
(VAAPI is Linux-only), but the editor referenced them unconditionally, so
macOS/Windows failed to compile (cannot find `encoder` in `gpu_video_encoder`).
Gate the zero-copy path (ZeroCopyVideo, try_build_zero_copy,
run_zerocopy_video_export, and the branches in start_video_export /
start_video_with_audio_export) behind cfg(target_os = "linux"). Factor the
software encoder-thread spawn into spawn_software_video so both the None arm
and the non-Linux path share it without duplication. Non-Linux always uses
the software path.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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