`lightcraft_codecs::gainmap`:
- `compute` measures a gain map (luminance or RGB, log2, block-averaged)
from an SDR and an HDR rendition; `apply` rebuilds the HDR rendition.
- ISO 21496-1 metadata: writer (fixed denominator) and reader (also the
common-denominator form; refuses unknown versions, backward maps and zero
denominators). Adobe `hdrgm` XMP reader and writer, per-channel values as
rdf:Seq; Apple's HDRGainMap/HDRToneMap description of the same map.
- `encode_jpeg`: base with CIPA DC-007 MPF index (gain map typed 0x050000),
the ISO marker and Container XMP merged into the user's packet, followed by
the gain map JPEG. `read_jpeg` / `is_gain_map_jpeg` find and decode the map.
Also: gain map images are no longer picked as embedded thumbnails (an Ultra
HDR JPEG could show its greyscale gain map in the grid). The JPEG APP-segment
building is shared with `encode_jpeg`. Fuzzed with the other decoders.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
skylar taylor-barrick