Request: Support API-4-style AVIF/HEIF gain-map encoding

libultrahdr currently supports API-4 for JPEG/R, allowing an application to provide an existing base image, gain-map image, and gain-map metadata for packaging without regenerating the gain map.

Could equivalent support be added for AVIF and HEIF output?

This would help applications such as libvips preserve an existing gain map when converting or saving an image. Without it, adopting libultrahdr v2 for AVIF/HEIF may require reconstructing HDR pixels and generating a new gain map, which is slower and can alter the original result.

Related broader transcoding request: [#398](https://github.com/google/libultrahdr/issues/398).

Start by locating the existing API-4 JPEG/R support and the AVIF/HEIF output paths. Compare how the supplied base image, gain-map image, and metadata are handled, and identify the public API and test coverage needed for equivalent support. Done means applications can package existing inputs for AVIF and HEIF without regenerating the gain map.

Written by the indexing model from the issue text.