test(socketpair): lock in domain-is-not-an-fd invariant (c00)

Audit of socketpair(2) found the tracing implementation already correct:
KindSocketpair captures the two output fds from the sv[2] buffer (args[3])
at exit and never treats args[0] (the address-family/domain constant) as a
file descriptor. Family=Network and UNCLASSIFIED ret are consistent with the
socket/accept siblings and the docs.

Add regression lock-in tests so a future field-shape or classification change
cannot silently regress to recording the domain integer as a bogus fd:

- TestClassifySocketpairNotFd: pins the name-based override so socketpair is
  KindSocketpair, never the generic KindFd path that reads args[0].
- TestHandleSocketpairExitDoesNotTrackDomainAsFd: uses AF_INET6 (10), distinct
  from the returned fds, and asserts fd 10 is never tracked while sv0/sv1 are.
- TestHandleSocketpairExitDropsFdsOnError: on ret!=0 no descriptors are tracked.

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

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