Refactor global variable access in VM and optimizer

Replaces `Rc<RefCell<Vec<Value>>>` with a new `GlobalStore` struct for
accessing global variables. This provides a more structured way to
manage global values and allows for future optimizations like sharing
immutable RTL portions across environments. The `GlobalStore` also
includes an `rtl_len` field to distinguish between RTL and user global
slots, enabling debug-time checks for writes to immutable RTL slots.

src/ast/compiler/optimizer/folder.rs

@@ -2,15 +2,15 @@ use crate::ast::nodes::{
     Address, AnalyzedNode, IdentifierBinding, Node, NodeKind, NodeMetrics,
 };
 use crate::ast::types::{Purity, RecordLayout, StaticType, Value};
-use std::cell::RefCell;
+use crate::ast::vm::GlobalStore;
 use std::rc::Rc;
 
 pub struct Folder<'a> {
-    pub globals: &'a Option<Rc<RefCell<Vec<Value>>>>,
+    pub globals: &'a Option<GlobalStore>,
 }
 
 impl<'a> Folder<'a> {
-    pub fn new(globals: &'a Option<Rc<RefCell<Vec<Value>>>>) -> Self {
+    pub fn new(globals: &'a Option<GlobalStore>) -> Self {
         Self { globals }
     }
 
@@ -94,7 +94,7 @@ impl<'a> Folder<'a> {
             NodeKind::Identifier {
                 binding: IdentifierBinding::Reference(Address::Global(idx)),
                 ..
-            } => self.globals.as_ref()?.borrow().get(idx.0 as usize)?.clone(),
+            } => self.globals.as_ref()?.get(idx.0 as usize)?,
             NodeKind::Constant(val) => val.clone(),
             _ => return None,
         };