Refactor bound node types for clarity

Introduce generic `CompilerPhase` trait to unify different stages of the
bound AST. Rename `LocalSlot` to `VirtualId` and introduce `StackOffset`
for clearer distinction between compile-time and run-time addressing.
Update type aliases and implementations to reflect these changes.

src/ast/compiler/optimizer/utils.rs

@@ -1,4 +1,4 @@
-use crate::ast::compiler::bound_nodes::{Address, AnalyzedNode, BoundKind, GlobalIdx};
+use crate::ast::compiler::bound_nodes::{Address, AnalyzedNode, BoundKind, GlobalIdx, VirtualId};
 use crate::ast::types::{Identity, Value};
 use crate::ast::vm::Closure;
 use std::collections::HashSet;
@@ -32,17 +32,17 @@ impl PathTracker {
 // --- UsageInfo ---
 #[derive(Default)]
 pub struct UsageInfo {
-    pub used: HashSet<Address>,
-    pub assigned: HashSet<Address>,
+    pub used: HashSet<Address<VirtualId>>,
+    pub assigned: HashSet<Address<VirtualId>>,
     pub used_identities: HashSet<Identity>,
 }
 
 impl UsageInfo {
-    pub fn is_assigned(&self, addr: &Address) -> bool {
+    pub fn is_assigned(&self, addr: &Address<VirtualId>) -> bool {
         self.assigned.contains(addr)
     }
 
-    pub fn is_used(&self, addr: &Address) -> bool {
+    pub fn is_used(&self, addr: &Address<VirtualId>) -> bool {
         self.used.contains(addr)
     }