Refactor: Rename TCO module to lowering

The TCO (Tail Call Optimization) module has been renamed to `lowering`.
This change better reflects the module's broader responsibility, which
includes not only TCO but also general AST transformations and
preparation for VM execution.

The `optimize` function has been renamed to `lower` to align with the
module's new name.

src/ast/environment.rs

@@ -15,10 +15,10 @@ use crate::ast::compiler::bound_nodes::{
 };
 use crate::ast::compiler::dumper::Dumper;
 use crate::ast::compiler::lambda_collector::LambdaCollector;
+use crate::ast::compiler::lowering::{ExecNode, Lowering};
 use crate::ast::compiler::macros::{MacroEvaluator, MacroExpander, MacroRegistry};
 use crate::ast::compiler::optimizer::Optimizer;
 use crate::ast::compiler::specializer::{FunctionRegistry, MonoCache, Specializer};
-use crate::ast::compiler::tco::{ExecNode, TCO};
 use crate::ast::rtl::intrinsics;
 use crate::ast::types::{
     Identity, NodeIdentity, Object, Purity, SourceLocation, StaticType, Value,
@@ -141,7 +141,7 @@ impl MacroEvaluator for RuntimeMacroEvaluator {
                 .join("\n"));
         }
 
-        let exec_ast = TCO::optimize(Analyzer::analyze(&typed_ast, &HashMap::new())); // Minimal analysis for macro eval
+        let exec_ast = Lowering::lower(Analyzer::analyze(&typed_ast, &HashMap::new())); // Minimal analysis for macro eval
 
         let mut vm = VM::new(self.global_values.clone());
         vm.run(&exec_ast)
@@ -586,8 +586,8 @@ impl Environment {
             .with_registry(self.typed_function_registry.clone());
         let optimized = optimizer.optimize(specialized);
 
-        // 5. TCO
-        TCO::optimize(optimized)
+        // 5. Lowering
+        Lowering::lower(optimized)
     }
 
     pub fn instantiate(&self, node: ExecNode) -> Rc<crate::ast::types::NativeFunction> {
@@ -705,14 +705,14 @@ impl Environment {
                     .with_purity(global_purity.clone());
                 let optimized_ast = optimizer.optimize(specialized_ast);
 
-                let tco_ast = TCO::optimize(optimized_ast);
+                let exec_ast = Lowering::lower(optimized_ast);
                 let mut vm = VM::new(global_values.clone());
-                let compiled_val = match vm.run(&tco_ast) {
+                let compiled_val = match vm.run(&exec_ast) {
                     Ok(v) => v,
                     Err(e) => return Err(format!("VM Error during specialization: {}", e)),
                 };
 
-                let ret_type = tco_ast.ty.ty.clone();
+                let ret_type = exec_ast.ty.ty.clone();
                 Ok((compiled_val, ret_type))
             },
         );
@@ -774,4 +774,3 @@ impl Environment {
         Ok((final_result, observer.logs))
     }
 }
-