HTTP-backed zarr store with per-chunk lazy fetch

Lazy: each read_bytes() issues a fresh HTTP GET (cached on disk if cache_dir is non-NULL). list_dirs() uses S3 ListObjectsV2 with the slash delimiter. Once a store is opened, the full object listing is fetched up front and memoised; subsequent exists() / list_dirs() checks hit the memoised manifest.

Usage

.zarr_store_http(url, cache_dir = NULL)