Add JPEG codec (v3)

Implements an array → bytes JPEG codec for the Zarr v3 codec pipeline, compatible with neuroglancer's precomputed "jpeg" chunk encoding.

• Encodes a uint8 chunk as a baseline JPEG image and decodes it back.
• The innermost chunk axis is treated as the channel axis: extent 3 → RGB, otherwise grayscale (1 channel).
• Configurable quality (0–100, default 90); validated on construction.
• Serializes to / deserializes from zarr.json ("name": "jpeg", "quality": …).

Notes / limitations

• Lossy — must not be used where exact values matter
• Only supports uint8; other dtypes are rejected with a clear error.
• Chunks exceeding JPEG's 65535-per-dimension limit are factored to fit, or rejected if no factorization works.