Remove unused TailCallRequest variant

The `TailCallRequest` enum variant and its associated logic have been
removed. Instead, the VM now uses an `Option<(Rc<dyn Object>,
Vec<Value>)>` field named `tail_call` to manage tail-call requests. This
change simplifies the `Value` enum and centralizes tail-call handling
within the `VM` struct.

src/ast/types.rs

@@ -259,7 +259,6 @@ pub enum Value {
     Function(Rc<NativeFunction>),
     Object(Rc<dyn Object>),   // For compiled Closures and other opaque types
     Cell(Rc<RefCell<Value>>), // Boxed value for captures
-    TailCallRequest(Box<(Rc<dyn Object>, Vec<Value>)>), // Internal: For TCO (Boxed to keep Value small)
 }
 
 impl PartialEq for Value {
@@ -280,9 +279,6 @@ impl PartialEq for Value {
             (Value::Function(a), Value::Function(b)) => Rc::ptr_eq(a, b),
             (Value::Object(a), Value::Object(b)) => Rc::ptr_eq(a, b),
             (Value::Cell(a), Value::Cell(b)) => Rc::ptr_eq(a, b),
-            (Value::TailCallRequest(a), Value::TailCallRequest(b)) => {
-                Rc::ptr_eq(&a.0, &b.0) && a.1 == b.1
-            }
             _ => false,
         }
     }
@@ -623,7 +619,6 @@ impl Value {
             Value::Function(_) => StaticType::Any, // Dynamic function
             Value::Object(o) => StaticType::Object(o.type_name()),
             Value::Cell(c) => c.borrow().static_type(),
-            Value::TailCallRequest(_) => StaticType::Any, // Internal state, but typable as Any
         }
     }
 }
@@ -667,7 +662,6 @@ impl fmt::Display for Value {
             Value::Function(f_meta) => write!(f, "<native fn, {:?}>", f_meta.purity),
             Value::Object(o) => write!(f, "<{:?}>", o),
             Value::Cell(c) => write!(f, "{}", c.borrow()),
-            Value::TailCallRequest(_) => write!(f, "<tail call request>"),
         }
     }
 }