rename `ArrayV3Metadata` and `ArrayV2Metadata`

[d-v-b]

proposed new names: ArrayMetadata_V3 + ArrayMetadata_V2, or ArrayMetadataV3 + ArrayMetadataV2. We could keep the old names around as aliases.

[maxrjones]

why?

[d-v-b]

logically, it's array metadata first, and then the version qualifies it. and for consistency: we are already using the X_V2 / X_V3 pattern elsewhere in the codebase to distinguish v2 and v3 metadata representations of comparable things.

[maxrjones]

I prefer ArrayMetadataV2 + ArrayMetadataV3; I find the _ is a mixing of naming styles.

https://github.com/zarr-developers/zarr-python/actions/runs/24095500618/job/70292885214?pr=3882 shows other inconsistencies.

[d-v-b]

i think we are allowed to mix styles when it favors readability: compare ArrayMetadataJSON_V3 and ArrayMetadataJSONV3

[maxrjones]

i think we are allowed to mix styles when it favors readability: compare ArrayMetadataJSON_V3 and ArrayMetadataJSONV3

Yeah, underscores between distinct sets of capital letters help readability. For the non-JSON class, I prefer ArrayMetadataV3.

We might want to jot down these preferred code styles in the contributors guide or CLAUDE.md