One supported stage-zero row of the logical index: the raw path bytes as
stored, the exact paired mode, the object in the declared namespace, and
the ordinary skip-worktree bit.
Parses one complete raw .git/index byte string under git-index-v1:
versions two through four, ordinary stage-zero rows only, the exact mode
pairings, strictly increasing prefix-free paths, mandatory unknown or
split-index and sparse-directory extensions rejected, and the trailing
checksum verified in the declared namespace.