test(rt_sigpending): lock in KindNull + FamilySignals + UNCLASSIFIED ret

Audit of rt_sigpending(2) confirmed the existing classification is correct
and added lock-in coverage:

- KindNull: int rt_sigpending(sigset_t *set, size_t sigsetsize). args[0] is
  a userspace output pointer to a sigset_t (a signal mask, not a traced I/O
  resource) and args[1] is the byte size; neither is an fd or path. The enter
  handler emits a null_event and must not capture either arg. Added
  TestGenerateRtSigpendingHandler with a negative assertion guarding against
  any ctx->args[] capture in the enter handler.
- Exit ret_type=UNCLASSIFIED: rt_sigpending returns 0/-1, a status code, not
  a byte count, so it must never be tagged READ/WRITE/TRANSFER. Added an exit
  handler assertion plus TestClassifyRetRtSigpendingUnclassified.
- FamilySignals: shares the family with the whole rt_sig* group plus
  kill/pause/sigaltstack/tkill/tgkill. Added lock-in family cases asserting
  every rt_sig* sibling alongside rt_sigpending in TestClassifySyscallFamily.

No classification/codegen/doc changes were required; mage generate produces
no diff. Full ./internal/... passes (only the known pre-existing flake
TestCleanupLeakedWorkloadTempDirCaughtByAssertion fails, unrelated).

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

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