[fixfon]

Fixes #15770

LineInfo::parse stored the detected line format in cached_line_format as soon as a line matched a parser, before the digest was validated. A free-text line such as 2024 release matched the single-space parser, was then rejected as improperly formatted, but had already made single-space the format for the rest of the file. Every following <digest> name line was read as the file name (with a leading space), so the files that were actually listed were never opened. With a file named a next to a tampered a, md5sum --check printed ' a': OK and exited 0. A line like Checksums for release did the same the other way round and dropped valid single-space lines.

process_checksum_line now parses into a local copy of the cache and writes it back only when the line is not improperly formatted. LineInfo::parse and its signature are unchanged. Lines that are well formed but fail later (DigestMismatch, missing or unreadable file) still set the format, as before.

Tests: three unit tests in validate.rs and one md5sum integration test, appended at the end of test_md5sum.rs. The two format-selection unit tests and the integration test fail on current main and pass with this change. The third unit test is a guard for the unchanged behaviour. I also ran the issue's example with a build from main and one with this change: before, md5sum --check reported the tampered files as OK (exit 0); after, it reports a: FAILED, c: FAILED and exits 1.

Ran locally: cargo fmt --all -- --check, cargo clippy --all-targets -- -D warnings at the workspace root, cargo test -p uucore --features checksum checksum::validate, and the test_md5sum, test_sha256sum and test_cksum integration tests (448 passed). Tested on macOS only; spell-check was not run.