AST Design

- shells can all ultimately output a single AST

- AST has span information, and is generic over "Name" type. A name type might start as a string, with passes resolving and type checking names

The issue names no files, tests, or entry points. Start by surveying the Rust project structure to find where shell parsing and AST types are defined. Done means an agreed design for one AST representation with span information and a generic Name type, but the scope is not specified here.

Written by the indexing model from the issue text.