fix: record spread works with named type aliases

Resolve type aliases (e.g. Player -> { pos: Vec2, speed: Float })
before checking if spread expression is a record type. Previously
{ ...p, field: val } failed with "must be a record type, got Player"
when the variable had a named type annotation.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/typechecker.rs

@@ -2569,6 +2569,7 @@ impl TypeChecker {
         // Start with spread fields if present
         let mut field_types: Vec<(String, Type)> = if let Some(spread_expr) = spread {
             let spread_type = self.infer_expr(spread_expr);
+            let spread_type = self.env.expand_type_alias(&spread_type);
             match spread_type {
                 Type::Record(spread_fields) => spread_fields,
                 _ => {