test(pipe): lock in pipe/pipe2 IPC classification and fd-pair exit reads

Audit of pipe(2)/pipe2(2) (task dx) confirmed the tracing implementation is
correct: KindPipe (not KindFd, since args[0] is an output ptr to int[2], not an
fd), FamilyIPC, and an UNCLASSIFIED int return. Enter stashes the output ptr
(flags=0 for pipe, args[1] for pipe2); exit reads the fd pair via
bpf_probe_read_user guarded by ret==0, mirroring the socketpair pipe-like
pattern. The only gaps were missing lock-in tests, now added:

- codegen: assert the exit handler reads the fd pair from the stashed output
  buffer (ret==0 guard, bpf_probe_read_user, fd0/fd1) and that the flag-less
  pipe variant hardcodes flags=0 and never reads args[1].
- classify: pipe/pipe2 are never KindFd and stay UNCLASSIFIED on ret.
- runtime: a failed pipe (ret==-1) tracks no descriptors and attaches no file.

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

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