Add check for trailing tokens

Ensure that a script does not contain any tokens after the main
expression. This prevents accidental trailing expressions and enforces a
clearer script structure.

src/ast/environment.rs

@@ -108,11 +108,16 @@ impl Environment {
         let mut parser = Parser::new(source)?;
         let untyped_ast = parser.parse_expression()?;
 
-        // 2. Bind & Type Check
+        // 2. Check for trailing tokens
+        if !parser.at_eof() {
+            return Err("Unexpected trailing expressions in script. Use (do ...) for sequences.".to_string());
+        }
+
+        // 3. Bind & Type Check
         let mut binder = Binder::new(self.global_names.clone());
         let bound_ast = binder.bind(&untyped_ast)?;
 
-        // 3. Execute
+        // 4. Execute
         let mut vm = VM::new(self.global_values.clone());
         vm.run(&bound_ast)
     }

src/ast/parser.rs

@@ -46,6 +46,10 @@ impl<'a> Parser<'a> {
         }
     }
 
+    pub fn at_eof(&self) -> bool {
+        matches!(self.current_token.kind, TokenKind::EOF)
+    }
+
     fn parse_atom(&mut self) -> Result<Node<UntypedKind>, String> {
         let token = self.advance()?;
         let identity = Rc::new(NodeIdentity { location: token.location });