Language: Records and field access MVP

[JROChub]

Context

• The current interpreter subset has basic types and control flow, but no structured data types.

Goal

• Add a minimal record type with field access to expand the language subset.

Proposal

• Syntax: record literal {x: 1, y: 2} and field access p.x.
• Update parser/AST, interpreter, and type checker.
• Add tests covering type checking, evaluation, and error cases.

Acceptance criteria

• Record literals parse and evaluate.
• Field access works with a clear runtime error for missing fields.
• Type checker infers/validates record field types.
• Tests pass (make test).