Extend the existing VideoDecoder with an optional hardware path, reusing its
demux/seek/keyframe/blob engine (no duplication):
- GpuVideoFrame (NV12 plane wgpu textures) + HwVideoImporter trait (editor
implements the DMA-BUF import; the AVFrame crosses as an opaque pointer so
core needn't reference the GPU crate) + HwDeviceHandle (opaque AVBufferRef).
- VideoManager::set_hardware_decode injects the VAAPI device + importer; each
decoder attaches hw_device_ctx + a get_format(VAAPI) callback before opening,
decodes into VAAPI surfaces, and imports them (no CPU copy).
- get_frame returns DecodedFrame::{Cpu,Gpu}; VideoFrame/VideoRenderInstance gain
an optional `gpu`. The frame cache budgets GPU frames as ~w*h*3/2 and keys on
whether the consumer wanted GPU.
- A hardware decoder serving a CPU consumer (export, render_hardware_ok=false)
downloads the surface via av_hwframe_transfer_data then swscales — so export
stays software/correct and only the preview goes GPU-resident. HW init or
import failure falls back to software per clip.
Dormant until the editor injects an importer (next): no importer => software,
unchanged.
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