Upgrade mode-3's 10-bit/HDR branch from "route to software libx264" (BUG 1's
behaviour) to a full-GPU pipeline: VAAPI decode → OpenCL tonemap (hable,
HDR10/HLG → bt709 SDR nv12) → h264_vaapi. Gen 9.5 (UHD 630) can't tonemap in
the VAAPI VPP (tonemap_vaapi fails), so OpenCL is the only HW route — proven on
the prod iGPU at ~4.2x realtime, ~1 CPU core.
- stream.py: `_select_encode` returns (hw, tonemap); a 10-bit/HDR HW-decodable
source picks the tonemap graph when the OpenCL runtime is present, else drops
to software. `_opencl_present` gates on a registered ICD. `_venc_opts` /
`_hwaccel_in_opts` gain the tonemap variant (named vaapi+opencl devices,
`-filter_hw_device ocl`, hwmap→tonemap_opencl→hwmap reverse). `_want_hw` kept
as the hw-half wrapper. The runtime HW→SW fallback still backstops a session
that emits no segment.
- Dockerfile: add the Intel NEO OpenCL runtime (intel-opencl-icd). It was removed
from Debian trixie, so it's pulled from bookworm (22.43 still supports Gen 9.5;
the .deb runs on the trixie base — verified live). Harmless without /dev/dri.
- route + FE: thread `tonemap` through the session response; the stats overlay
mode row reads "transcode · GPU · HDR→SDR" for the tonemap path.
- tests: tonemap command branch, the (hw, tonemap) selection matrix (auto/on ×
8-bit/10-bit × opencl), start_session tonemap wiring, and the fmtMode tag.