Module expressions 

Expression resolution - transforms name references into indexed lookups.

Each expression variant is handled in Resolver::resolve_expression:

• Identifier -> ResolvedIdentifier { depth, slot } if found in scope, left as Identifier if not (stdlib names, unresolved at this stage)
• Assign -> ResolvedAssign { depth, slot, value }
• Lambda -> ResolvedLambda { capture_depth, ... } with a fresh scope for its parameters; capture_depth is the scope stack length at the point of definition, used by the evaluator to capture the environment
• Call with a single-segment path -> CallExpr with a ResolvedIdentifier callee if the name is in scope; left as Call for stdlib paths
• All other variants recurse into their sub-expressions unchanged

All resolution mutates nodes in place via ExprId + Arena::get_mut. The node’s ExprId never changes - only its kind does. Child ExprIds embedded in a parent’s kind stay valid across the rewrite since resolving a child mutates the arena slot it already occupies, not its address.