fixes v2 metadata

[lazarusA]

closes #12

[gemini-code-assist]

Code Review

This pull request implements on-the-fly synthesis of .zmetadata for unconsolidated Zarr v2 stores, enabling hierarchy discovery for HTTP clients without requiring directory listings. It also introduces path traversal protection and support for HEAD requests. Review feedback identifies critical runtime issues, specifically that the 3-argument call to Zarr.consolidate_metadata and the access to Zarr.JSON will likely fail. Additionally, the reviewer suggested including Content-Length in HEAD responses for synthesized metadata and optimizing JSON serialization by removing indentation.