JPEG implementation and vectorization

JPEG runtime code remains safe Rust. The production kernel boundary is src/codecs/jpeg/kernels.rs, with exact scalar reference behavior and safe wide arithmetic where enabled. jpeg-wide-color is an opt-in color-conversion candidate; its existence is not a general performance claim.

Evaluate a change using the complete public API benchmark, including odd dimensions, quality, chroma sampling, progressive/optimized modes, and grayscale/CMYK. A faster inner loop can lose end-to-end after allocation, conversion, or routing costs. Keep context/output lifetime inside both measured implementations' boundaries.

Do not import architecture-specific unsafe intrinsics, unchecked pointer loads, uninitialized output buffers, or fixture-specific fast paths to claim parity. Preserve exact fixed-point rounding, saturation, and byte output with maintained Pillow cases. Document the earliest reference divergence beside a subtle fix.

Historical rejected candidates and their local measurement diaries remain in Git history. Current evidence comes from the maintained same-host benchmark, source-bound reports, and the full fixture matrix.