add jpeg codec and testcases

[konstibob]

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.

[normanrz]

We'll also discuss this extensions PR, but my main feedback is that the jpeg codec should not change the data that is passed in upon encoding. It should throw errors, if incompatible data is being written. Otherwise, it should rely on other codecs (e.g. transpose, reshape) to make the data compatible with what jpeg can store.