test(select): lock in nfds/timeval capture and UNCLASSIFIED exit

Audit of syscall select confirmed the tracing is already correct:
select is KindPoll/FamilyPolling like poll/ppoll/pselect6, the enter
handler captures nfds from args[0] as a count (not as an fd) and the
timeout from the args[4] timeval, and the exit is an UNCLASSIFIED
ret_event (ready-fd count, not a byte transfer).

Add TestGenerateSelectHandlerCapturesNfdsAndTimevalTimeout mirroring the
ppoll lock-in test, with negative assertions that no argument is ever
captured as an fd and that the exit carries no bytes/fd fields. This
guards against regressing nfds (a count) into a KindFd fd capture.

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

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