test(retbytes): assert read byte counts for pread64/preadv/preadv2

The retbytes integration coverage exercised read/write/sendto/etc but the
positional read p-variants only had presence assertions (pread64) or no
coverage at all (preadv/preadv2), so their READ_CLASSIFIED byte accounting
was validated only by unit tests, not end-to-end.

Add a positive byte-count assertion to TestReadwritePread and new
readwrite-preadv / readwrite-preadv2 workload scenarios plus integration
tests that read a known payload and assert the attributed byte count,
mirroring the existing pwrite64 assertion. preadv2 lacks a Go
syscall.SYS_PREADV2 constant, so its number is provided per-GOARCH
(amd64=327, arm64=286) following the securitySyscallNumbers pattern.

Addresses the read side of b20.

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

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