test(timerfd): assert timerfd_settime/gettime fd capture end-to-end

The fd-from-air-eventfd-users workload only called timerfd_create then
closed the fd, so timerfd_settime/timerfd_gettime were never exercised
by any integration test. Commit 6ac9fa4 fixed those two syscalls to
KindFd@arg0 (capturing the operating timerfd via fd_event instead of a
null_event), but that fix had no end-to-end coverage.

Extend the workload to arm the still-open timerfd via timerfd_settime
(1s relative expiry, so it never fires) and read it back via
timerfd_gettime before closing. Assert in TestFdFromAirEventfdUsers that
both enter handlers fire (MinCount>=1) and resolve to the "timerfd:"
path prefix, proving arg0 fd is captured rather than null. Locks in the
6ac9fa4 KindFd fix.

splice/tee are NOT touched: retbytes_test.go already asserts
enter_splice/enter_tee plus positive transfer byte counts, which
inherently exercises their arg0 fd capture, so no new coverage is
needed there.

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

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