test(setpgid): lock in KindNull/Process/UNCLASSIFIED audit findings

Audit of setpgid(pid_t pid, pid_t pgid): both args are process/process-
group identifiers (kernel tracepoint type pid_t), never an fd or path;
the call returns int 0/-1. Verified it is correctly classified KindNull
(null_event), FamilyProcess, and UNCLASSIFIED ret, and that the Process
and null lists in docs/syscall-tracing-plan.md stay in sync. No
classification change was needed.

Add lock-in tests so a future stray reclassification trips immediately:
- TestClassifySetpgidNullEnter feeds the REAL tracepoint fields (pid_t
  pid, pid_t pgid) and asserts KindNull, proving args[0] (pid) is never
  mistaken for an fd: isFdType matches only int/unsigned int/unsigned
  long (not pid_t) and the fd heuristic also requires field name fd.
- TestClassifyExitSetpgidUnclassifiedRet asserts the exit is KindRet and
  ClassifyRet stays UNCLASSIFIED (status code, not a byte count).
- Add setpgid to the retclassify UNCLASSIFIED cluster beside setsid/getsid.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

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