[Benchmark] add truncate

[duncan020313]

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

Question	Answer
Where is the GNU code?	truncate.c: main / do_ftruncate
What must I implement?	Explicit -c -s N with absolute nonnegative decimal size through signed 64-bit maximum; stable regular files/symlinks, directories and missing paths.
What must the spec guarantee?	Missing-path no-create success; otherwise typed FilesystemTruncate request, absolute size and operand order. Ordinary open/path errors use cannot-open wording; no late truncate/close faults in this scope.
Which APIs can I use?	Typed filesystem request/result calls selected by this entry; path/kind queries where needed, output outcomes and diagnostics.
What is outside this task?	Default file creation, reference/relative/block sizes, nonregular objects, resource/late operation/close faults and injected failures.

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

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