test(generate): lock in tkill classification (Signals/null/UNCLASSIFIED)

Audit of tkill(2) (task 310) confirmed correct tracing: tkill(tid, sig)
is FamilySignals, kind=null, ret UNCLASSIFIED, matching its siblings
kill/tgkill/rt_sig*. tkill/tgkill are intentionally absent from the
name-only kind table; ClassifyFormat returns KindNone for them (the
pid_t tid is not matched by the fd rule, so the thread id is never
misread as a file descriptor) and classifyEnterForGeneration promotes
that to KindNull at generation time.

This was untested, so add lock-in coverage closing the gap:
- TestGenerateTkillHandler: enter emits null_event, captures no arg
  (tid is not an fd), exit reports raw status as UNCLASSIFIED.
- TestClassifyTkillFallsThroughToNull: pins ClassifyFormat=KindNone and
  the KindNull generation fallback, so a future fd-rule regression fails.
- TestClassifyRetTkillUnclassified: 0/-1 status is not a byte count.
- Extend TestClassifySyscallFamily with kill/tkill/tgkill (enter+exit)
  so a stray reclassification out of FamilySignals trips the test.

No generated output or runtime behavior changed (mage generate clean).

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

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