test(signalfd4): lock in args[3] flags index for eventfd handler

Audit of signalfd4(2) confirmed the tracing is correct: classified as
KindEventfd in FamilyIPC like its fd-creating siblings (eventfd2,
timerfd_create, inotify_init1, signalfd), flags captured from args[3]
per signalfd4(ufd, mask, sizemask, flags), and the return value left
Unclassified (it is an fd, not a byte count).

Add testdata fixtures FormatSignalfd4/FormatExitSignalfd4 (real Linux 7.0
tracepoint data) and a codegen lock-in test asserting the generated
handler reads flags from args[3], with negative guards against args[0]
(ufd), args[1] (mask pointer) and args[2] (sizemask).

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

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