Feat: Add global inlining and DCE

Introduce global inlining by allowing the Optimizer to access the
environment's global values. This enables replacing global variable
accesses with their constant values when appropriate.

Additionally, implement Dead Code Elimination (DCE) for global
definitions. A global definition can be removed if it was only used for
inlining within the current script and has no side effects.

Update the Optimizer struct to hold an optional reference to the global
values and modify the `Optimizer::new` constructor to accept this. The
`Environment::specialize` and `Environment::compile_script` methods are
updated to pass the global values to the optimizer.

The `SubstitutionMap` is also updated to track script-local global
substitutions and used global indices, supporting both global inlining
and global DCE.

src/ast/environment.rs

@@ -182,7 +182,7 @@ impl Environment {
         let specialized = self.specialize_node(node);
 
         // 2. Optimize (Level 1: Cracking, Level 2: Collapsing)
-        let optimizer = Optimizer::new(self.optimization_level);
+        let optimizer = Optimizer::new(self.optimization_level).with_globals(self.global_values.clone());
         let optimized = optimizer.optimize(specialized);
 
         // 3. TCO (Always performed, converts to ExecNode)
@@ -227,7 +227,7 @@ impl Environment {
                 let specialized_ast = sub_specializer.specialize(retyped_ast);
 
                 // 3. Optimize (Phase 2: Cracking & Folding)
-                let optimizer = Optimizer::new(opt_level);
+                let optimizer = Optimizer::new(opt_level).with_globals(global_values.clone());
                 let optimized_ast = optimizer.optimize(specialized_ast);
 
                 // 4. TCO (converts to ExecNode)