Refactor scope handling and immutability

Introduces `fixed_scope_idx` to `Binder` and `Environment` to track
immutable scopes.
This allows enforcing that definitions (`def`) cannot occur in scopes
that are considered fixed or immutable, such as the global scope or the
initial bootstrap scope.
The `FunctionCompiler` is updated to use `fixed_scope_idx` and `is_root`
to determine scope immutability.

src/ast/environment.rs

@@ -75,6 +75,7 @@ pub struct Environment {
     pub global_types: Rc<RefCell<HashMap<GlobalIdx, StaticType>>>,
     pub global_purity: Rc<RefCell<HashMap<GlobalIdx, Purity>>>,
     pub global_values: Rc<RefCell<Vec<Value>>>,
+    pub fixed_scope_idx: i32,
     pub function_registry: Rc<RefCell<GlobalFunctionRegistry>>,
     pub typed_function_registry: Rc<RefCell<GlobalAnalyzedRegistry>>,
     pub monomorph_cache: Rc<RefCell<MonoCache>>,
@@ -112,6 +113,7 @@ struct RuntimeMacroEvaluator {
     global_names: Rc<RefCell<HashMap<Symbol, (GlobalIdx, Identity)>>>,
     global_types: Rc<RefCell<HashMap<GlobalIdx, StaticType>>>,
     global_values: Rc<RefCell<Vec<Value>>>,
+    fixed_scope_idx: i32,
 }
 
 impl MacroEvaluator for RuntimeMacroEvaluator {
@@ -127,7 +129,7 @@ impl MacroEvaluator for RuntimeMacroEvaluator {
         }
 
         let mut diag = Diagnostics::new();
-        let (bound_ast, captures) = Binder::bind_root(self.global_names.clone(), node, &mut diag)?;
+        let (bound_ast, captures) = Binder::bind_root(self.global_names.clone(), self.fixed_scope_idx, node, &mut diag)?;
         let bound_ast = crate::ast::compiler::captures::CapturePass::apply(bound_ast, &captures);
         let checker = TypeChecker::new(self.global_types.clone());
         let typed_ast = checker.check(&bound_ast, &[], &mut diag);
@@ -161,6 +163,7 @@ impl Environment {
             global_types: Rc::new(RefCell::new(HashMap::new())),
             global_purity: Rc::new(RefCell::new(HashMap::new())),
             global_values: Rc::new(RefCell::new(Vec::new())),
+            fixed_scope_idx: -1,
             function_registry: Rc::new(RefCell::new(HashMap::new())),
             typed_function_registry: Rc::new(RefCell::new(HashMap::new())),
             monomorph_cache: Rc::new(RefCell::new(HashMap::new())),
@@ -172,6 +175,9 @@ impl Environment {
             loaded_modules: Rc::new(RefCell::new(HashSet::new())),
         };
         crate::ast::rtl::register(&env);
+        
+        let mut env = env;
+        env.fixed_scope_idx = 0;
 
         // Automatically add standard search paths (CWD and CWD/rtl)
         if let Ok(cwd) = std::env::current_dir() {
@@ -212,6 +218,7 @@ impl Environment {
             global_names: self.global_names.clone(),
             global_types: self.global_types.clone(),
             global_values: self.global_values.clone(),
+            fixed_scope_idx: self.fixed_scope_idx,
         };
         MacroExpander::new(self.macro_registry.borrow().clone(), evaluator)
     }
@@ -423,7 +430,7 @@ impl Environment {
         };
 
         let (bound_ast, captures) =
-            match Binder::bind_root(self.global_names.clone(), &expanded_ast, diagnostics) {
+            match Binder::bind_root(self.global_names.clone(), self.fixed_scope_idx, &expanded_ast, diagnostics) {
                 Ok(res) => res,
                 Err(e) => {
                     diagnostics.push_error(e, None);