test(generate): lock in msgctl SysV-op classification

msgctl(2) operates on a System V message queue identified by msqid
(args[0]). msqid is a SysV IPC id returned by msgget, NOT a file
descriptor; capturing it as an fd would corrupt the fd-resource view.
ior already classifies msgctl as KindSysVOp in FamilyIPC (consistent
with siblings semctl/shmctl/msgget/msgsnd/msgrcv), emits a null_event
with no arg capture, and reports the int status as UNCLASSIFIED.

Add lock-in tests pinning this behavior:
- TestGenerateMsgctlHandler: KindSysVOp for msgctl/semctl/shmctl, all
  FamilyIPC; generated enter handler emits null_event and captures no
  args (asserts no ctx->args[ and no ev->fd); exit ret is UNCLASSIFIED.
- TestClassifyRetMsgctlUnclassified: msgctl ret is never a byte count.

No implementation change needed; mage generate produces no diff.

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

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