read_superblock_extension_in, cache_image_state_in, CacheImage::decode_in
and CacheImage::block_in take &dyn Storage (whose length is the end of
file); the image block is one bounded read. The &[u8] functions are
wrappers; applying an image in place still needs the bytes in memory.
New test: extension messages, a cache image and a corrupt one decode to
the same results through a read_at-only CountingStorage.
Co-Authored-By: Claude Opus 5.5 (1M context) <[email protected]>