test(retbytes): assert readlinkat READ_CLASSIFIED byte count end-to-end

readlink/readlinkat are READ_CLASSIFIED (exit ctx->ret = link-target byte
count), but the integration suite only asserted the enter_readlinkat
pathname tracepoint via MinCount in link_test.go. The exit byte
classification and positive duration were never validated end-to-end,
unlike sibling READ-classified syscalls (read/recvfrom/getxattrat/
getdents64) in retbytes_test.go.

Add retbytesReadlinkat to the phase-A workload: it creates a symlink with
a known non-empty absolute target, opens the parent O_DIRECTORY, and
re-issues SYS_READLINKAT in a short spaced window (mirroring the
getdents64 driver) so ior can attach and capture an enter/exit pair under
parallel load. Each call re-resolves the same link, so ctx->ret stays
equal to the target length and is strictly positive.

Add readlinkat (and symlink, used to build the link without mixing
tracepoints) to retbytesTraceArgs, assert enter_readlinkat presence
(MinCount) plus bytes>=1 via assertEventBytesAtLeast and a positive
duration. bytes>=1 (not an exact target length) because the resolved
path varies across temp dirs; >=1 is the safest invariant.

Coverage hardening only; classify.go readlink/readlinkat=ReadClassified
and the BPF arg capture (args[1]=pathname for readlinkat) are correct.

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

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