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/analyzer.rs

@@ -1,5 +1,5 @@
 use crate::ast::compiler::bound_nodes::{
-    Address, AnalyzedNode, BoundKind, GlobalIdx, NodeMetrics, TypedNode,
+    Address, AnalyzedNode, BoundKind, GlobalIdx, Node, NodeMetrics, TypedNode, TypedPhase,
 };
 use crate::ast::types::Purity;
 use std::collections::{HashMap, HashSet};
@@ -318,7 +318,7 @@ impl<'a> Analyzer<'a> {
             BoundKind::Error => (BoundKind::Error, Purity::Impure),
         };
 
-        crate::ast::compiler::bound_nodes::Node {
+        Node {
             identity: node.identity.clone(),
             kind: new_kind,
             ty: NodeMetrics {
@@ -334,7 +334,7 @@ trait NodeExt {
     fn for_each_child<F: FnMut(&TypedNode)>(&self, f: F);
 }
 
-impl NodeExt for BoundKind<crate::ast::types::StaticType> {
+impl NodeExt for BoundKind<TypedPhase> {
     fn for_each_child<F: FnMut(&TypedNode)>(&self, mut f: F) {
         match self {
             BoundKind::If {