test(classify): lock in pwritev2 WRITE byte-count classification

Audit of pwritev2(2) confirmed its tracing is already correct: enter is
KindFd (fd at args[0]), return is WRITE_CLASSIFIED so the byte count is
counted as written like its pwritev/writev/write/pwrite64 siblings, and
family is FS. Add a dedicated lock-in test pinning kind, family, and the
WRITE return classification, with the read-side preadv2 as a contrast and
the whole p/readv/writev family asserted together to guard against a
stray off-by-one reclassification.

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

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