Remove is_tail field from Call node

The `is_tail` field on the `BoundKind::Call` node was an intermediate
representation for tail-call optimization and is no longer needed as a
distinct field. The TCO pass now embeds this information directly into
the `RuntimeMetadata` of the `ExecNode`, which is the VM's internal AST.
This simplifies the `BoundKind::Call` structure and removes redundant
information.

src/ast/compiler/specializer.rs

@@ -48,9 +48,9 @@ impl Specializer {
 
     fn visit_node(&self, node: TypedNode) -> TypedNode {
         let (new_kind, new_ty) = match node.kind {
-            BoundKind::Call { callee, args, is_tail } => {
+            BoundKind::Call { callee, args } => {
                 let (new_callee, new_args, ret_ty) = self.specialize_call_logic(*callee, *args, node.ty.clone());
-                (BoundKind::Call { callee: Box::new(new_callee), args: Box::new(new_args), is_tail }, ret_ty)
+                (BoundKind::Call { callee: Box::new(new_callee), args: Box::new(new_args) }, ret_ty)
             },
 
             // Recursive traversal for other nodes