Silence CodeQL warning for BLAKE2b byte offset

[zhongkechen]

Summary

• express the BLAKE2b byte offset through array subscripting
• preserve the existing byte-wise addressing and hash behavior
• avoid CodeQL's suspicious pointer-add-with-sizeof pattern

This addresses code-scanning alert #1, which is a false positive because the underlying pointer is byte-sized.

Validation

• Ubuntu 24.04 / GCC 13 release build with benchmarks enabled
• ctest --test-dir build --output-on-failure (3/3 passed)
• git diff --check