[Benchmark] add unlink

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

Question	Answer
Where is the GNU code?	unlink.c: main
What must I implement?	Exactly one operand, including symlink operands.
What must the spec guarantee?	One FilesystemUnlink request without terminal dereference; preserve the returned state and select GNU failure diagnostic.
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?	Recursive removal and open detached handles.

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

Resolved by #39.

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