prohibit specifying computeds that type doesn't have declared

`MyType.select(computed=...)` should be prohibited at the type level if `MyType` doesn't have `computed`. We won't be able to unpack fields we don't have definitions for, so we have to simply reject this at the typing level.

Start by tracing the Python typing for MyType.select and how computed definitions are represented. Add type-level rejection for computed names absent from the selected type, then verify typing coverage for MyType.select(computed=...) with and without a declared computed.

Written by the indexing model from the issue text.