Rename the `Array.read_only` method?

`Array.read_only` _looks_ like a boolean predicate, asking whether the current array is read-only or not.

_Instead_ it's a _method_ that converts the current array into a read-only array.

```
def read_only(self) -> Array: # returns an Array
def is_sharded(self) -> bool: # actual predicate, same class
```

I'll leave this as-is for 0.1 and we can come back to discuss

Start by reviewing the Array.read_only method and the nearby is_sharded method to compare their naming and behavior. Confirm whether a replacement name is agreed on and check the API implications before making any change; done means the conversion method has an unambiguous non-predicate name and existing callers remain consistent.

Written by the indexing model from the issue text.