test(generate): lock in pwritev WRITE byte-count classification

Audit of pwritev(2) confirmed the existing classification is correct:
pwritev returns the number of bytes written, so its exit is
WRITE_CLASSIFIED (matching write/pwrite64/writev/pwritev2), fd is at
args[0] (KindFd), and it lives in the FS family. The read-side sibling
preadv stays READ_CLASSIFIED. No implementation changes were needed.

Add TestClassifyPwritevWriteByteCount as a lock-in test mirroring the
prior pwritev2/pwrite64 audits, with a preadv off-by-one contrast guard
and transfer/unclassified negative checks across the whole p/readv/writev
family so any stray reclassification trips the test.

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

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