Fix Utf8Transcoder AVX2 bug, add SIMD boundary tests
- Added Hungarian language preference rule to copilot-instructions.md.
- Fixed AVX2 SIMD bug in Utf8Transcoder: corrected upper-half store offset from Vector128<ushort>.Count to Vector256<ushort>.Count, preventing memory overlap on 32+ byte ASCII runs.
- Added Utf8TranscoderTests covering all SIMD/scalar paths, with boundary and round-trip tests for ASCII, Hungarian, CJK, emoji, and mixed content, ensuring correctness and BCL compatibility.
