[Benchmark] add sha512sum

[duncan020313]

Add the sha512sum benchmark, including its specification, implementation, proof, and GNU comparison tests, within the scope below.

Question	Answer
Where is the GNU code?	cksum.c: digest_file / main (algorithm selected by src/local.mk)
What must I implement?	Digest generation; -b, -t, -z; ordered file/stdin operands.
What must the spec guarantee?	SHA-512 word equations, initial state, padding, bit-length encoding and digest truncation; exact GNU record escaping and file marker.
Which APIs can I use?	ReadFileWithOutcome / ReadStdinWithOutcome, output outcomes and diagnostics. Digest arithmetic is contributor-owned; no trusted hash answer primitive.
What is outside this task?	Check-file verification and tagged output.

Source paths in the table are relative to coreutils/src/.

See implementation notes, common implementation rules, and the contribution workflow.