test(kill): lock in KindNull enter and UNCLASSIFIED ret

Audit of kill(2) (pid_t pid, int sig): the pid at args[0] is a process/
process-group identifier and sig a signal number, neither an fd nor a
path, so the enter tracepoint is KindNull and the int 0/-1 return is a
status code (UNCLASSIFIED), not a transferred byte count. Classification
and docs (Signals/null) already matched and need no change.

Add TestClassifyExitKillUnclassifiedRet (the return-value lock-in its
signal siblings tkill/tgkill/rt_sigqueueinfo already have) and harden
TestClassifyKillExplicitNull to assert no PathnameField is captured,
documenting the deliberate contrast with pidfd_send_signal (KindFd/IPC,
args[0] is a real pidfd).

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

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