Idempotent setkv Capstone: Make a Config Writer Safe and Repeatable (Graded by Running It)

The tool s9setkv FILE KEY VALUE must ensure the line KEY=VALUE appears in FILE exactly once — replacing an existing KEY= line or appending a new one — and running it again with the same arguments must change nothing. The starter version is fragile. Harden it; you are graded by the grader running your script against adversarial inputs. See /root/brief.txt for the path and contract.

Make it satisfy all six properties:

1. Validate args — wrong argument count or missing FILE → non-zero exit, no change.

2. Append when absent — a new key adds exactly one KEY=VALUE line.

3. Update when present — an existing key is replaced; no duplicate KEY= lines.

4. Idempotent — running twice leaves exactly one KEY= line.

5. Preserve others — unrelated lines stay intact.

6. Literal value — a value with /, &, or spaces (e.g. /etc/foo bar) is stored exactly (beware sed replacement strings; a literal printf write is safer).

Keep the same path and keep it executable.