Signed/unsigned accounting: int64_t delta added to uint64_t ac_bytes can wrap

Summary

vfs_stdaccount takes int64_t delta(:149), adds to uint64_t ac_bytes(:160) and per-chunk uint64_t space(:168-169). Negative delta exceeding counter wraps to near UINT64_MAX. vq_write_ok additions(:447,:463,:479) can also wrap. Wrapped counter -> write DoS or limit bypass. Gated by vfs_quota_enabled.