Module evaluator 

Core evaluator - expression evaluation, function calls, and the runtime state.

Structs

Evaluator

The tree-walking interpreter, carrying all runtime state.

PItem

A slot in the environment - holds a value, its declared type, and mutability.

Enums

EnvironmentItem

A single environment entry. Currently only PItem exists; the enum wrapper leaves room for future variants (e.g. closures, records).