Investigate unchecked cast in NDArray.asType()

Investigate whether this cast can produce a ClassCastException at runtime and either add proper type checking or document why the cast is safe.

Start at the NDArray.asType() entry point and inspect the unchecked cast and the types it handles. Run the relevant existing tests, if available, to determine whether a ClassCastException is possible. Done means adding type checking or documenting a demonstrated safety rationale.

Written by the indexing model from the issue text.