`encode_avif_pq`: linear Rec. 2020 with SDR white at 1.0 → PQ (SDR white at
203 cd/m², BT.2408), BT.2020 NCL YCbCr 4:4:4, 10-bit, with CICP nclx
(9/16/9, full range) and a clli box. ravif fixes the sRGB curve, so this
drives the rav1e + avif-serialize it already uses (no new crates).
Export: AVIF with `hdr` renders the HDR edit in Rec. 2020 and writes PQ.
Photos without an HDR edit export as before, in every format. CoreImage
decodes the file as HDR (same 3.1 % of the frame above SDR white as the gain
map JPEG of the same edit).
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
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