fix![zarr-metadata]: define JSONValue type, and narrow object annotations to use JSONValue

[d-v-b]

This defines a JSONValue type that models python objects that easily become JSON. To be clear, json.loads is still essentially untyped, so this JSONValue type requires type narrowing when you read actual JSON data. But for constructing values that will fit into JSON, this is a useful type and necessary for modelling e.g. the fill_value field of array metadata documents, which previously were object, and that's far too open.

Happy to be bikeshedded on the name of the type -- JSON or JSONValue. I think JSONValue is more accurate, since JSON can denote a file or a bytestream or the format.

This is a breaking change to zarr-metadata, so it needs a minor version bump (e.g., to 0.3.0) when we issue our next release of the package.

[Copilot]

Pull request overview

This PR tightens the typing surface of zarr-metadata by introducing a recursive JSONValue type and using it to replace overly-broad object annotations in v2/v3 metadata TypedDicts (notably fill_value, attributes, and extension/configuration fields). It also narrows the struct field data_type to the existing MetadataFieldV3 union.

Changes:

• Add JSONValue (recursive JSON-encodable value type) and apply it across v2/v3 metadata schemas in place of object.
• Narrow several schema fields to domain-specific types (e.g., StructField.data_type -> MetadataFieldV3).
• Add a .claude hook log file to the repository (likely unintended for source control).

Reviewed changes

Copilot reviewed 10 out of 10 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
packages/zarr-metadata/src/zarr_metadata/_common.py	Introduces JSONValue and uses it for NamedConfig.configuration.
packages/zarr-metadata/src/zarr_metadata/v3/array.py	Replaces object with JSONValue for fill_value, attributes, and v3 extension extra-items.
packages/zarr-metadata/src/zarr_metadata/v3/group.py	Narrows v3 group attributes to Mapping[str, JSONValue].
packages/zarr-metadata/src/zarr_metadata/v3/data_type/struct.py	Narrows StructField.data_type to MetadataFieldV3 and StructFillValue to JSONValue.
packages/zarr-metadata/src/zarr_metadata/v3/codec/scale_offset.py	Narrows offset/scale config fields to JSONValue.
packages/zarr-metadata/src/zarr_metadata/v3/codec/cast_value.py	Narrows scalar_map entries to tuple[JSONValue, JSONValue].
packages/zarr-metadata/src/zarr_metadata/v2/array.py	Narrows v2 array fill_value and merged attributes to JSONValue.
packages/zarr-metadata/src/zarr_metadata/v2/group.py	Narrows v2 group merged attributes to JSONValue.
packages/zarr-metadata/src/zarr_metadata/v2/codec.py	Narrows v2 codec extra-items to JSONValue.
packages/zarr-metadata/src/zarr_metadata/v2/attributes.py	Narrows .zattrs content to Mapping[str, JSONValue].
packages/zarr-metadata/.claude/hooks/.logs/hook-log.jsonl	Adds a hook execution log entry to the repo.

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