Scalar variables from kerchunk refs get manifest shape (1,) instead of ()

[TomNicholas]

Kerchunk uses v2-style chunk key "0" for scalar variables. get_chunk_grid_shape interprets this as 1D shape (1,) instead of scalar (), because only "c" is recognized as scalar.

This causes ManifestStore.get_entry to return an ndarray instead of a string when resolving scalar chunk paths, which crashes urlparse downstream.

Reproducer

from virtualizarr.manifests.manifest import ChunkManifest

manifest = ChunkManifest(
    entries={"0": {"path": "s3://bucket/file.nc", "offset": 100, "length": 4}},
    shape=(),
)

Raises IndexError: too many indices for array: array is 0-dimensional, but 1 were indexed

In practice this manifests when using open_virtual_dataset with loadable_variables on a file containing scalar coordinate variables (e.g. time, step).