[Benchmark] add sum

[duncan020313]

Add the sum 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; sum.c: bsd_sum_stream / sysv_sum_stream
What must I implement?	Default BSD checksum; -r and -s; ordered file/stdin operands.
What must the spec guarantee?	BSD rotate/add congruences or System V byte-sum folding; ceiling block count with the selected 512/1024-byte unit and filename rules.
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?	Other cksum algorithms and check-file syntax.

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

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