Refactor: Use Value::Closure enum variant

Previously, compiled closures were stored as `Value::Object` and then
downcasted. This commit introduces a dedicated `Value::Closure` enum
variant to represent compiled closures. This improves type safety and
simplifies handling of closures throughout the compiler and VM.

This change involves:
- Updating type definitions to use `Value::Closure`.
- Adjusting downcasting logic to directly access the `Closure` struct.
- Modifying `run_with_args` and `run_with_args_observed` to expect
  `Value::Closure` for tail-call targets.
- Refining the handling of closures in the `Optimizer` and `Inliner` for
  better type clarity.

src/ast/compiler/optimizer/engine.rs

@@ -3,7 +3,7 @@ use crate::ast::nodes::{
     IdentifierBinding, LambdaBinding, Node, NodeKind, UpvalueIdx, VirtualId,
 };
 use crate::ast::types::{Purity, Value};
-use crate::ast::closure::Closure;
+
 use std::cell::RefCell;
 use std::rc::Rc;
 
@@ -343,9 +343,9 @@ impl Optimizer {
                         }
                     }
 
-                    if let NodeKind::Constant(Value::Object(ref obj)) = callee_opt.kind
+                    if let NodeKind::Constant(Value::Closure(ref closure_rc)) = callee_opt.kind
                         && path.inlining_depth < 5
-                        && let Some(closure) = obj.as_any().downcast_ref::<Closure>()
+                        && let closure = closure_rc.as_ref()
                         && (closure.upvalues.is_empty()
                             || closure.function_node.ty.purity >= Purity::SideEffectFree)
                         && !closure.function_node.ty.is_recursive

src/ast/compiler/optimizer/inliner.rs

@@ -2,7 +2,7 @@ use crate::ast::nodes::{
     Address, AnalyzedNode, IdentifierBinding, NodeKind, VirtualId,
 };
 use crate::ast::types::{Purity, Value};
-use crate::ast::closure::Closure;
+
 use std::cell::RefCell;
 use std::collections::HashSet;
 use std::rc::Rc;
@@ -35,12 +35,8 @@ impl<'a> Inliner<'a> {
             | Value::Keyword(_)
             | Value::Record(_, _)
             | Value::DateTime(_) => true,
-            Value::Object(obj) => {
-                if let Some(closure) = obj.as_any().downcast_ref::<Closure>() {
-                    closure.upvalues.is_empty() && !closure.function_node.ty.is_recursive
-                } else {
-                    false
-                }
+            Value::Closure(closure_rc) => {
+                closure_rc.upvalues.is_empty() && !closure_rc.function_node.ty.is_recursive
             }
             _ => false,
         };

src/ast/compiler/optimizer/utils.rs

@@ -3,7 +3,7 @@ use crate::ast::nodes::{
     NodeKind, VirtualId,
 };
 use crate::ast::types::{Identity, Value};
-use crate::ast::closure::Closure;
+
 use std::collections::HashSet;
 
 // --- PathTracker ---
@@ -56,12 +56,10 @@ impl UsageInfo {
                 self.collect(value);
             }
             NodeKind::Constant(v) => {
-                if let Value::Object(obj) = v
-                    && let Some(closure) = obj.as_any().downcast_ref::<Closure>()
-                {
+                if let Value::Closure(closure_rc) = v {
                     self.used_identities
-                        .insert(closure.function_node.identity.clone());
-                    self.collect(&closure.function_node);
+                        .insert(closure_rc.function_node.identity.clone());
+                    self.collect(&closure_rc.function_node);
                 }
             }
             NodeKind::Identifier { binding, .. } => {