Superblock::parse_in(&dyn Storage, offset) reads one bounded window of
128 bytes (the largest superblock is 100) and runs the existing version
parsers on it; parse and refresh_eof keep their &[u8] signatures as
wrappers. No behaviour change: on a file longer than the window no bounds
check can fail, and on a shorter one the window is the whole file. New
test: every version and truncation parses to the same result through a
read_at-only CountingStorage, in one read.
Co-Authored-By: Claude Opus 5.5 (1M context) <[email protected]>