test(sigaltstack): lock in KindNull/Signals classification and UNCLASSIFIED ret

Audit of sigaltstack(2) confirmed the existing tracing is correct:
both args are userspace stack_t pointers (new + old alternate signal
stack), neither an fd nor a path, and it returns 0/-1. It is already
classified KindNull in FamilySignals across classify.go, family.go,
docs/syscall-tracing-plan.md, and the generated C/Go artifacts, matching
the rt_sig*/kill/pause sibling group. No discrepancies were found.

Add lock-in tests mirroring the prior rt_sigpending audit:
  - TestGenerateSigaltstackHandler asserts the enter handler emits a
    null_event capturing no args and the exit handler reports ctx->ret
    as UNCLASSIFIED.
  - TestClassifyRetSigaltstackUnclassified asserts the 0/-1 status is
    never tagged as a byte count.

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

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