test(retbytes): assert getdents64 READ_CLASSIFIED byte count end-to-end

getdents64 was listed in retbytesTraceArgs but the retbytes workload
never issued it and TestRetbytesPhaseA never asserted its exit byte
count, leaving the READ_CLASSIFIED ctx->ret path for getdents/getdents64
unverified end-to-end (unlike sibling read/recvfrom/getxattrat).

Add retbytesGetdents to the phase-A workload: it populates a temp
directory with several files, opens it O_DIRECTORY, and re-issues
getdents64 in a short window (lseek-rewind each iteration) so ior can
attach and capture an enter/exit pair under parallel load. A non-empty
directory guarantees ctx->ret > 0.

Assert enter_getdents64 presence (MinCount) plus bytes>=1 via
assertEventBytesAtLeast and a positive duration, mirroring the existing
READ-classified siblings. Bytes>=1 (not an exact payload length) because
dirent size varies with filename length and alignment.

Coverage hardening only; classify.go getdents/getdents64=ReadClassified
is correct.

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

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