SIMD Utf8Transcoder.GetUtf8ByteCount + test suite
Introduced SIMD-accelerated Utf8Transcoder.GetUtf8ByteCount for efficient UTF-8 byte counting, replacing all writer-side Encoding.UTF8.GetByteCount usages. Added 29 unit tests for correctness across ASCII, Hungarian, CJK, emoji, and boundary cases. Updated benchmarks to ensure FixStr is bypassed and wire mode is selectable. Documented implementation and dead-code review in BINARY_TODO.md. No public API changes.
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.
Loretta