Refactor: Use root_purity instead of global_purity

This commit refactors the purity analysis from using a
`HashMap<GlobalIdx, Purity>` to a `Vec<Purity>` indexed by
`GlobalIdx.0`.

This change simplifies the data structure and makes purity lookups more
efficient. The `Binder`'s `globals` field has also been removed as it
was not being used.

src/ast/compiler/macros.rs

@@ -627,7 +627,6 @@ mod tests {
     use crate::ast::compiler::Binder;
     use crate::ast::parser::Parser;
     use crate::ast::types::{Object, Value};
-    use std::cell::RefCell;
 
     struct SimpleEvaluator;
     impl MacroEvaluator for SimpleEvaluator {
@@ -783,7 +782,7 @@ mod tests {
 
         let mut diag = crate::ast::diagnostics::Diagnostics::new();
         // fixed_scope_idx = 0 means scope 0 is frozen (Global)
-        let result = Binder::bind_root(initial_scopes, 1, 0, Rc::new(RefCell::new(HashMap::new())), &expanded, &mut diag);
+        let result = Binder::bind_root(initial_scopes, 1, 0, &expanded, &mut diag);
         assert!(
             result.is_ok(),
             "Should find global '*' Error: {:?}",
@@ -807,7 +806,7 @@ mod tests {
         let expanded = expander.expand(untyped).unwrap();
 
         let mut diag = crate::ast::diagnostics::Diagnostics::new();
-        let result = Binder::bind_root(vec![], 0, 0, Rc::new(RefCell::new(HashMap::new())), &expanded, &mut diag);
+        let result = Binder::bind_root(vec![], 0, 0, &expanded, &mut diag);
         assert!(result.is_ok(), "Hygiene failed: {:?}", result.err());
     }
 
@@ -827,7 +826,7 @@ mod tests {
         let expanded = expander.expand(untyped).unwrap();
 
         let mut diag = crate::ast::diagnostics::Diagnostics::new();
-        let result = Binder::bind_root(vec![], 0, 0, Rc::new(RefCell::new(HashMap::new())), &expanded, &mut diag);
+        let result = Binder::bind_root(vec![], 0, 0, &expanded, &mut diag);
         assert!(
             result.is_ok(),
             "Explicit Hygiene with Backticks failed: {:?}",