With HDR output on, a photo edited in HDR exports as a 16-bit PNG with the PQ
curve in Rec. 2020 (SDR white at HDR_REFERENCE_WHITE_NITS), described by a
`cICP` chunk (PNG third edition: BT.2020 primaries, PQ, RGB, full range) and
a `cLLI` content light level chunk (the levels the HDR AVIF records). EXIF,
XMP and print resolution are kept; no ICC profile next to cICP. Photos without
an HDR edit, and HDR edits with HDR output off, export as before.
`codecs::encode_png_pq` sits next to `encode_avif_pq` and takes the same
input. Follow-up to #646, ported from #466.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
