Make the dormant core HW-decode engine live for the preview path:
- hw_video.rs: editor's HwVideoImporter — maps a decoded VAAPI surface to a
DRM-PRIME DMA-BUF and imports it as wgpu NV12 plane textures on the *shared*
device (the only one with the import extensions). install() creates the VAAPI
device and injects it + the importer into the VideoManager.
- main.rs: track whether the shared device is actually in use; only then (Linux,
not LB_NO_SHARED_DEVICE) install hardware decode, using the CreationContext's
shared device + adapter.
- nv12_blit.rs + nv12_blit.wgsl: NV12 plane textures → BT.709 → sRGB-encoded →
linear, written straight into the Rgba16Float HDR layer (no CPU upload). Colour
math mirrors the software path so HW/SW video match; honours full_range.
- stage.rs: the preview Video arm branches on inst.gpu (NV12 blit) vs rgba_data
(existing upload+blit_straight); sets render_hardware_ok = !cpu_renderer so the
CPU fallback still gets software frames.
- video_exporter.rs: sets render_hardware_ok(false) before both compositing
passes — export composites on the encoder's separate device, so a hardware
decoder downloads to CPU instead (export stays software, correct).
- dmabuf.rs: imported plane textures now also carry SAMPLED/TEXTURE_BINDING so
they can be sampled by the NV12 blit (they were render-target-only); into_planes
hands the textures to the longer-lived GpuVideoFrame.
- video.rs: cache-key the GPU/CPU representation on want_gpu (HW-configured AND
render_hardware_ok) so software-only decode keeps a single cache entry.
Preview only this pass; export GPU-residency is the 3c-export follow-up. Untested
at runtime here (no GPU/display in container) — both crates compile.
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